“Incessant “ means continuing without a pause or interruption.
Option A is the correct answer because “constant” means something that doesn’t change with time, here the rain is constant and is not stopping, hence it is the correct answer.
Option B is incorrect because “heavy” means ‘ a lot of it, in large quantities” which is incorrect because that isn’t not the correct meaning for incessant.
Option C is incorrect because “intermittent” means occurring at irregular intervals, not constant, which makes it the antonym of incessant.
Option D is incorrect because “unexpected” means something that comes as a surprise which is not the meaning for the given word.