plain subject verb agreement question.
Revenues is plural so you need a plural verb.
also note a source of income needs to modify business travel.

(A) a source of income without which most commercial airlines could not survive, are [Bingo!]

(B) a source of income without which most commercial airlines could not survive, is [ subject verb agreement error]

(C) and most commercial airlines use it as a source of income to survive with, are [ you dont need a conjunction here. You need a noun modifier and not a conjunction. Eliminated!]

(D) which is a source of income which is needed by most commercial airlines who could not survive without it, [ verb itself is missing in this sentence. Absurd!]

(E) are which most commercial airlines use as a source of income without which they are unable to survive, is [ Subject verb agreement error!]

Hope this helps!

komal wrote:Declining revenues resulting from a decrease in business travel, a source of income without which most commercial airlines could not survive, are going to force many commercial airlines to increase prices and decrease services in the coming months.

(A) a source of income without which most commercial airlines could not survive, are
(B) a source of income without which most commercial airlines could not survive, is
(C) and most commercial airlines use it as a source of income to survive with, are
(D) which is a source of income which is needed by most commercial airlines who could not survive without it,
(E) are which most commercial airlines use as a source of income without which they are unable to survive, is

OA A Source kaplan 800

B - Subject verb agreement (subject - revenues , verb -is)
C - conjunction usage
D & E wordy unncessary repetitive pronouns
A remains (proper SVA)

komal wrote:Declining revenues resulting from a decrease in business travel, a source of income without which most commercial airlines could not survive, are going to force many commercial airlines to increase prices and decrease services in the coming months.

(A) a source of income without which most commercial airlines could not survive, are
(B) a source of income without which most commercial airlines could not survive, is
(C) and most commercial airlines use it as a source of income to survive with, are
(D) which is a source of income which is needed by most commercial airlines who could not survive without it,
(E) are which most commercial airlines use as a source of income without which they are unable to survive, is

OA A Source kaplan 800

My pick is A.
B is a Trap. is
C - it is ambiguous.
D and E are awkward