Rather than trying to set this up with your own equation, try just using the answer choices. Using (A), for example:

If the 1996 C:T ratio was 1:2, then cars revenue could have been 100, and trucks revenue could have been 200.

We are told that cars revenue went down 11%, so the 1997 revenue was 89.

We are told that trucks revenue went up 7%, so the 1997 revenue was 214.

We are also told that the total revenue went up 3%. If we add the two 1997 revenues together, we get 303, which is indeed 1% higher than the total 1996 revenue of 300.

Thus, we know we have the correct answer.

the total revenue increased by 1%, not 3%...

No need to test numbers:

we know that :
-cars decreased by 11% so the rev. is (89/100)x
-trucks increased by 7% so the rev is (107/100)y
-cars and trucks increased by 1% so the rev. is (101/100)(x+y)

So:
(89/100)x + (107/100)y = (101/100)(x+y)
(89/100)x + (107/100)y = (101/100)x + (101/100)y
(6/100)y = (12/100)x
y = 2x

Since we're being asked for the ratio of car sales to truck sales, we want to find x/y....
x/y = 1/2