Correct Answer: reason

Solution : The correct choice is the third option.

The most suitable word to fill in the blank is reason.

In this context, "for no reason" means without a specific cause or explanation. The sentence conveys that the person didn't understand why the laughter occurred; it was spontaneous and not triggered by any discernible cause.

The meanings of the other options are as follows:

Just: While "for no just" might make sense grammatically, it doesn't convey the intended meaning in the context of the sentence.

Disbelief: This doesn't fit the context because the sentence is describing adoration for the way he laughed, not expressing doubt or disbelief.

Account: "For no account" doesn't convey the same sense of spontaneity or lack of a specific cause as the phrase "for no reason" does.

Therefore, the most suitable word is reason as it fits seamlessly into the sentence and maintains the intended meaning.