The correct answer is "few":
The university usually has few students attending from other countries.
"Few" is a quantifier that is used with plural countable nouns, in this case, "students"
"Less" is incorrect since it is used with uncountable nouns (for example milk)
"Lesser" is also used for uncountable nouns so it does not apply to the sentence.
And fewer is also incorrect since it is the comparative of few and in this sentence no comparison is made.
