△abc is reflected to form △a′b′c′ . the vertices of △abc are a(−7, 1) , b(−5, −3) , and c(−3, 2) . the vertices of △a′b′c′ are a′(−7, −1) , b′(−5, 3) , and c′(−3, −2) . which reflection results in the transformation of △abc to △a′b′c′ ?

The vertices of the preimage and final image are as follows 
A(−7, 1) ---> A′(−7, −1)
B(−5, −3) ---> B′(−5, 3)
C(−3, 2)  ---> C′(−3, −2)
it can be noted that whilst the x coordinates remain the same in the final image, y coordinate values, the sign changes.
eg: A (x = -7) and A' (x = -7)
therefore x coordinates haven't changed the value nor sign 
 A (y = 1) and A' (y = -1)
y coordinates have been multiplied by -1 hence changing the sign.
the reflection goes as (x,y) = (x,-y) 
this is a reflection over the x axis.