Option A - 'Custom' means a tradition or an act followed by the people of a specific region, religion or a nation. It is a noun and not a verb. Hence, option A is incorrect.

Option C - 'Customarily' means the way in which one follows a custom. It also means 'usually'. It is an adverb and not a verb. Hence, option C is incorrect.

Option D - 'Customer' is a person who purchases goods or services. 'Customer' is a noun and not a verb. Hence, option D is incorrect.

Option B - 'Accustom' means to make someone or something accept something and be used to it/or a person. 'Accustom' is an action, hence it is a verb. Thus option B is the correct answer.

ACCUSTOM is the verb among all the given options.