Sporadic means occurring at irregular intervals or only in a few places; scattered or isolated.
Hence option C is the correct answer because “irregular” means something that doesn’t occur at regular intervals or on a normal basis, hence it is the correct meaning of the given word.
Option A is incorrect because “frequent” means to occur or happen many times in short intervals which is in fact the antonym of sporadic.
Option B is incorrect because “irksome” means annoying or irritation and is not related to the given word.
Option D is incorrect because “sharp” means in a sudden or abrupt way, hence it is incorrect.