Answer:
The answer is B, a semicolon.
Mr. Gomez made sure we understood fractions; we then began algebra.
Explanation:
A sentence uses a semicolon when it does not have the word "and", "but", "because", etc.
In this example, if it were to say "Mr Gomez made sure we understood the fractions and we then began algebra", It would need a comma.
Because this sentence continues without that word "and", it needs a semicolon. This is because it is two separate sentences within one sentence.
I hope this helps :)
