Question

find the error with subject - verb agreement. select the incorrect verb and type it correctly.
the doppler effect results from the fact that as a moving object approaches
an observer, the apparent frequency of its sound waves increase.

Explanation:

The subject of the clause "the apparent frequency of its sound waves increase" is "the apparent frequency", which is a singular noun phrase. So the verb "increase" (used for plural subjects or first/second person singular in present simple) is incorrect. The correct verb should be "increases" to agree with the singular subject.

Answer:

Incorrect verb: increase; Correct verb: increases