Step 1: Understanding the Concept:
This is a "Second Conditional" or "Hypothetical Conditional" sentence. It describes an imaginary or impossible situation in the present or future.
Step 2: Detailed Explanation:
In the "if" clause of a hypothetical condition, we use the subjunctive mood.
For the verb "to be", the standard rule in formal English is to use "were" for all subjects (I, you, he, she, it, we, they), even if the subject is singular.
The main clause follows with "would + base verb".
"If I were you" implies "I am not you, but if I were...".
Step 3: Final Answer:
The correct word is "were". Therefore, option (C) is correct.