Concept:With "too", the correct pattern is: too + adjective + a/an + noun.
Explanation:The word "too" is followed by an adjective before the article.
We need the order: adjective "nice" then article "a", then noun "person".
So the correct phrase is "too nice a person".
The complete sentence is: He is too nice a person to say no.
Option A is wrong because an article is required before "person".
Option B gives the wrong order after "too".
Option D does not fit the required grammatical pattern.
Therefore, the blank must contain "nice a".
Answer:C. nice a