“_________ boys are there?” “there are two boys.”

A. How much
B. How many
C. Why
D. Which

If a noun is an uncountable noun (which is often in singular form), the ‘much’ determiner should be used. On the other hand, the determiner ‘many’ should be used with countable nouns, or plural nouns. “Boys” is a countable noun.
“How many boys are there?” “there are two boys.”