Select the most appropriate option to fill in the blank.
No ______ than fifty trees were uprooted in the storm.

The blank is followed by a countable plural noun ("fifty trees") and the comparative word "than".
Let's check the rules for countable vs. uncountable comparison:

Fewer is used with plural countable nouns (e.g., trees, books, people).

Less is used with uncountable nouns (e.g., water, sand, money).

Since "trees" are countable and numbered ("fifty"), the comparative determiner fewer must be used. "No fewer than fifty trees..." means exactly fifty or more trees were uprooted.