The correct answer A
Explanation :-
First find the premise and conclusion :-

Premise 1) AN airline named "Consolidate airline" ordered - 20 TX MODEL JETS from another aircraft maker called "Manto"
Premise 2) But "Consolidated airline" cancelled more expensive- 20 Z SERIES JETS from Manto. (in net effect Manto lost the expensive order and got a less expensive order)
Conclusion) Less order for Manto Aircrafts airliner means Manto will have fire employee

So Now we have to weaken the conclusion by showing that MANTO will not fire employee. The option that shows that Manto will not fire employees will be correct

A shortage of orders for Manto Aircraft's airliners has led analysts to predict that the manufacturer will have to lay workers ,the new order that consolidated airlines has just made for 20 of Manto's model TX jets does not provide a reason for the analysts to revise their predictions ,because simultaneously with its new order , consolidated canceled its existing order for an equal number of Manto's larger, more expensive model Z jets

Which of the following ,if true, most seriously weakens the argument?

A. Manto relies more heavily on outside subcontractors to provide the labor to manufacture the Model Z than it does the Model TX
Keep it for later

B. The Manto employees who currently work to manufacture the model Z are Not proficient at manufacturing the model TX
Wrong:- So employees cannot make Model TX . But now Manto have to produce model TX because order of model Z is cancelled. And these employees cannot make Model TX. Imagine even if Manto fires these employees, it will strengthen the conclusion and not weaken it. We have to show manto will not fire employee. This option suggests Manto will fire employee. Hence Wrong

C. Manto includes all required maintenance work for the first five years in the price of each jet it sells .
Wrong:- So what, Do they tell us that series Z needed more maintenance than TX model. May be TX model requires a lot of maintainance, may be not. What is the Amount of money that MANTO receives by maintenance . None of these is mentioned in question stem.

D. Manto has had to lay off workers several times within the past ten years but has typically rehired many of the workers when it subsequently received new orders.
Wrong:- So MANTO is a good company and rehire workers in the future. But Do we know for sure that Manto will receive subsequent new orders and will be able to rehire the fired workers. We can't say for sure .. so this option is wrong. Also the question is about firing the workers in current time. What happens in future is irrelevant.

E. A large number of the airliners in "consolidated fleet" are at the beginning of their expected service life.
Wrong:- A large number of planes of "CONSOLIDATED AIRLINES" will start to need maintenance. OK !! so what ! How does it affects MANTO. At worst this options make us assume that maintenance work will be done by other company and not Manto and thus Manto will not gain anything. At best this options tells us that Since CONSOLIDATED AIRLINE is already buying 20 TX model jet , it might not need to maintain its old aircrafts.

BAMM.. we are done and we don't have a clear answer except A.. SO by the process of elimination we found out that A id the answer.
NOW LETS SEE HOW OPTION A CAN BE THE CORRECT ANSWER

A. Manto relies more heavily on outside subcontractors to provide the labor to manufacture the Model Z than it does the Model TX
RIGHT :- Since now Manto does not have to make MODeL Z. It will not heavily use subcontractors from outside. Since it will not heavily use subcontractors from outside, it will not have to pay them lot of money. Since it will not pay outside people, that lot of money will be saved, since lot of mmoney will be saved,that money might be used to pay salaries of employees. Hence Monty will not fire employees. A is correct.