Question

on the one hand conservationists and many wildlife biologists assert that the losses have been severe and in some cases even catastrophic.
in this sentence, assert means to:
state categorically
hint at the meaning of something
judge to be probable
prove to be false

Explanation:

"Assert" means to state something with confidence and firmness. "State categorically" also implies a definite and firm statement. "Hint at the meaning of something" is too vague. "Judge to be probable" is about likelihood, not firm assertion. "Prove to be false" is the opposite of what "assert" means.

Answer:

state categorically