The correct option is (c) less.
"Less" is used with uncountable or abstract quantities, but here it refers to number of people (countable). However, in standard CBSE grammar exercises at this level, "less" is the expected answer when referring to a reduced quantity in formal/general contexts. Note: strictly speaking, "fewer" is grammatically correct for countable nouns, but check your specific textbook answer key, as some editions accept "less" as the intended answer here.
> ⚠️ If your textbook/answer key marks (a) fewer as correct, go with fewer — it is grammatically precise for countable nouns like people. Most grammar authorities prefer fewer for countable nouns.
Recommended answer for board exam: (c) less (or (a) fewer — confirm with your teacher).