The underlined verb is becoming because when you look at the is becoming you see that it is in present tense and active voice. The is carries the present tense and the becoming carries the progressive aspect.
The underlined verb in the sentence "the growth of suburbs is becoming an important factor in the decline of urban areas." adopts the form of present progressive.
The present progressive is formed by combining the verb “to be” with the present participle. In this case we have the verb IS plus the present participle of the verb become. The present participle is merely the “-ing” form of a verb: becoming.
