The correct answer is D. no article.
The principal of a school is a specific person, so we do not use an article. We would say “The principal of my school is called Radha.” or “Radha is the principal of my school.”
We use articles to introduce general nouns, not specific nouns. For example, we would say “I saw a dog.” or “I saw the dog.” In the first sentence, we are not talking about a specific dog, just any dog. In the second sentence, we are talking about a specific dog that we have seen before.
In the sentence “The principal of my school is called Radha,” we are talking about a specific principal, not just any principal. Therefore, we do not use an article.