The correct option is
B accident
The abstract noun form of the adjective, 'accidental' is 'accident'. 'Accidental' means happening by chance or unexpectedly. The related word, 'accident' means: An event which happens by chance or that which has no deliberate cause. Hence, option B is the correct answer.
Option A is incorrect as the word indicated there is the adjective form itself.
Option C: The word, 'acceptance' is an abstract noun which means: General agreement that something is satisfactory or right. It also means the act of agreeing to an offer or invitation. As the meaning of this word is quite different from the word, 'accidental', this is not the correct answer.
Option D: The word, 'accent' is a noun which means: A distinctive way of pronouncing a language, especially one associated with a particular country, area, or social class. As the meaning of this word is quite different from the word, 'accidental', this is not the correct answer.