The correct answer is (C) has.
"The number of road accidents has increased this year." ('The number of' takes a singular verb.)
The subject here is "the number," which is singular, so it requires the singular verb "has." Do not be misled by "road accidents" (plural) — it is part of a prepositional phrase, not the subject. "Are" and "were" are incorrect forms entirely; "have" is plural and wrong here.