The context refers to Bob being absent due to some reason. Here, past participle form of verb will be used. 
"Been" in option A is the past participle of 'be' which indicates existence. This is incorrect.
"Went" in option B is incorrect as the context refers to "He's or he has". This is not used with 'went' and the preposition 'to'. 
"Gone" in option C is correct as it indicates the past participle of the verb 'go'. This used with the preposition 'to' indicates the direction of the action. Thus, we can surmise that Bob is unavailable because he has gone to work. This is the correct answer.
"Being" in option D is incorrect as it is a gerund which can be used as a noun to indicate present participle or existence. 
Option C is the correct answer.