Take a look at the item again:
Mackenzie had to leave the room, (A)
for her father was noisily crunching raw
radishes, (B) which produced
so annoying a sound (C)
that studying was impossible.
You wanted to do this:
McKenzie had to leave the room, for her father was noisily crunching raw
radishes, which produced so annoying a sound; that
studying was impossible.
Adding a semicolon after sound makes a fragment. That studying was impossible is a subordinate clause. You cannot connect a subordinate clause with such a strong mark of punctuation.
You might want to review the rules.
Go back to the sentence to try again.