The correct answer is C. very.
The word “very” is the most appropriate word to describe the level of sadness in the sentence “He was very sad yesterday.” This is because “very” is an adverb that means “to a great extent or degree.” In this case, it is being used to describe the extent to which the person was sad.
The other options are not as appropriate because they do not convey the same level of sadness. The word “fairly” means “to a moderate extent or degree.” The word “much” means “a large amount or number.” The word “too” means “in excess of what is required or desirable.”
In conclusion, the correct answer is C. very.