Question

Yesterday was the ......(hot)day of the year.​

Answer 1

HEY MATE HERE IS YOUR ANSWER

Yesterday was the hottest day of the year

HOPE IT HELPS YOU MATE AND PLEASE MARK ME AS BRAINLY MATE

Answer 2

Yesterday was the hottest day of the year.​

1. the word hottest in the above sentence is an adjective of superlative degree.
2. there are three degree of comparison of adjectives- positive degree, comparative degree and superlative degree.
3. we use superlative degree to describe the noun in its extreme degree.
4. the word hottest describe the noun, day as the extreme hot of the year.
5. positive degree is used in simple form and comparative degree is used  when we compare two nouns.

hence, the correct option is hottest.