Step 1: Read the sentence as a whole.
\textit{“If I was you, I would not attend the function.”}
Step 2: Identify the grammatical rule involved.
In conditional sentences expressing an imaginary or unreal situation, the verb “were” is used for all subjects, including I.
Step 3: Locate the error.
The phrase “If I was you” is incorrect.
Correct form:
\[
\text{If I were you, I would not attend the function.}
\]
Step 4: Final conclusion.
The error lies in part \(\boxed{\text{(A)}}\).