Select the most appropriate option to fill in the blank.

My mother is the ____ inspiration in my life.

biggest

The most appropriate option to fill in the blank is biggest.

The sentence is about the importance of the mother in the speaker's life. The mother is the speaker's biggest inspiration, which means that she is the most important person who has inspired the speaker.

The other options are not as appropriate.

• Big and bigger are not as strong as biggest. They do not accurately convey the speaker's feelings about their mother.
• Most biggest is grammatically incorrect. It should be "the biggest".