Find the part of the given sentence that has an error in it. If there is no error, choose 'No error.'
I know how hardest you have tried to fit in my shoes.

The correct answer is (c) I know how hardest.
Explanation:
The word "hardest" is a superlative and should not be preceded by "how." The correct form should be "how hard" (comparative) or simply "hard."
Corrected sentence:
"I know how hard you have tried to fit in my shoes."