Select the most appropriate word to fill in the blank.

Father didn't believe ______ I said.

what

The most appropriate word to fill in the blank is what.

The word "what" is used to refer to something that has been said or done. In this sentence, the speaker is saying that the father did not believe what they said.

The other options are not as appropriate.

• The word "which" is used to refer to a specific thing or thing out of a group of things. It is not clear how the father did not believe which of the things the speaker said.
• The word "who" is used to refer to a person or people. It is not clear how the father did not believe who the speaker was.
• The word "that" is used to refer to something that has been said or done. However, it is not as specific as the word "what."