In order to select the verb from the given options, let's understand each words' meaning.
Option B - 'Retention' means a continued use of someone or something, it also means continued possession of someone or something. It is a noun and not a verb. Hence, option B is incorrect.
Option C - 'Retainer' is a fee that is paid to secure someone's service, it also means a thing or person that holds something or someone in place. It is a noun and not a verb. Hence, option C is incorrect.
Option D - 'Retainable' is a person who can be retained (secured to give service or for other purposes). It is an adjective and not a verb. Hence, option D is incorrect.
Option A - 'Retain' means to keep possession of someone or something. 'Retain' is an action, hence it is a verb. Thus, option A is the correct answer.
RETAIN is the verb among the given options.