Step 1: Analyzing the sentence.
The phrase "I am proud .......... my son" requires a preposition that shows the relationship between the speaker's feeling of pride and their son. The appropriate preposition to use here is "of". Step 2: Identifying the correct answer.
- (A) to is incorrect because "proud to" does not fit in the context of the sentence.
- (B) with is incorrect because "proud with" is not commonly used in this context.
- (C) of is correct because "proud of" is the standard expression used to indicate pride in someone or something.
- (D) from is incorrect because "proud from" does not fit grammatically in this context. Step 3: Conclusion.
Thus, the correct answer is (C) of.