Answer:
B
Explanation:
You can try to solve each:
A.

add 31 to both sides

subtract 2x from both sides

This means infinite solutions
B.

add 31 to both sides

add 2x to both sides

divide both sides by 4
This does have one solution
C.

subtract 31 from both sides

subtract 2x from both sides

This is not true, so no solutions
D.

add 2 to both sides

subtract 2x from both sides
0 = -29
not true, so b is the answer.