The answer should provide the reason why John will get on the plane to New York 30 mins later even though his first plane arrives on time:

(A) John's airline is known for always being on time- the answer should provide the reason why John will get on the plane to New York 30 mins later even though his first plane arrives on time--- this option just restates what is written in the passage doesn't provide a reason- wrong!

(B) a number of other passengers on John's first flight were also scheduled to take John's connecting flight- this option just states that along with john other passengers will probably get on the plane 30 mins later as well.... however we do not know the reason why they are all boarding 30 mins later..--wrong!

(C) at the airport in Chicago, airlines always fly into and out of the same terminal--- this is a possible explanation as it takes time for the first flight to take off and the second flight to show up!!(30 mins is the buffer time)!- correct

(D) John knew there was another flight to New York scheduled for one hour after the connecting flight he was scheduled to take-- other flights are out of scope!- wrong!

(E) the airline generally closes the doors of a particular flight 10 minutes before it is scheduled to take off---- this option doesn't prove why the passengers will take 30 mins to get on the plane in the first place. we are concerned about the boarding time and not about when the doors of the flight close- wrong!