Answer:
C
Explanation:
Well anything with an exponent not equal to one for both x and y is not linear. That means that c is not equal to 0 as well.
D: is not true. x + y = 4 is linear. The coefficients are both 1.
B: is false because x has a power of 2 on it. y = 2^x is not linear either, but that is not relevant in this question.
A: See the remark above. The statement is true, but it does not make the equation linear.
I would pick C, but the question is very poorly worded. A is hard to get rid of. It is simply not what would make an equation linear.