At least some students are in the habit of doing their exercises in a slovenly manner.

Answer: Option A

Slovenly : (especially of a person or their appearance) untidy and dirty.
Neat : arranged in a tidy way; in good order.
Proper : strictly so called; in its true form.
Decent : conforming with generally accepted standards of respectable or moral behaviour.
Correct : free from error; in accordance with fact or truth.

Antonym of Slovenly is Neat