........................ 1983 ............... 1993

Ashtrays .......... 40 ....................... >= 51 (Increase will be more than 10)

Umbrella .......... 60 ....................... 70 (+10 from before)

Already note that say 60 increases by 10 so that is an increase of 16%.
Now 40 must increase by more than 10 so by more than 25%. This %age increase will be higher because 40 is less than 60. When you increase 40 by the same amount as you increase 60, the percentage increase to 40 will be more. When you increase 40 by a higher amount than 60, the percentage increase in 40 will be even more!

Which must be true? (Refer to the data taken above)

A. In 1993, the percentage of Consolidated Metal's production devoted to ashtrays was greater than that in 1983.

Say 40 and 60 are both increased by the same amount 10. In 1993, production of ashtrays of the total will be 50/120 = 42% (higher than before). So if ashtrays are actually increased by a greater amount (more than 10), they will account for an even higher percentage of the total.
Hence, this will be true in all cases. When a smaller number is increased by a greater amount, it will take a larger share.

B. In 1993, Consolidated Metals produced fewer ashtrays than umbrella stands.

We don't know. Perhaps they produced 51 ashtrays in 1993 or perhaps they produced 101.

C. In 1993, ashtrays were more cost-efficient to produce than they were in 1983.

Not known. Argument doesn't talk about cost of production.

D. The production of umbrella stands required more labor in 1993 than did ashtrays in 1983.

Again, irrelevant.

E. In 1993, Consolidated Metals produced more ashtrays than umbrella stands.

We don't know. Perhaps they produced 51 ashtrays in 1993 or perhaps they produced 101.

Answer (A)