Which one of the following is grammatically correct?

Option C is the correct answer.

This uses proper indirect speech structure with correct tense agreement and verb usage. "Told" is followed by a person and a reported clause, and the past tense "was" matches the reporting verb.

The other options either misuse "said to," mix tenses incorrectly, or lack proper structure and punctuation. "Said to" requires a direct quote or needs to be rewritten with a reporting clause using "that".