The acute angles of a right triangle are?
a) supplementary.
b) complementary.
c) congruent.
d) linear.

Well, in a right triangle, one angle is 90 degrees.

Since the total of all of the angles are 180, you can subtract 90 from that since you only need the two acute angles:
180-90=90

Now you know that the two angles will add up to 90 no matter what each is. So, they are complementary, or B.

Hope this helped!