The correct option is
C That
In order to fill in the blank with the correct personal pronoun, let us understand what personal pronouns are. We use personal pronouns in place of a particular thing or a person's name/noun. For example: 'I spoke to them'. Here, 'I' is a subjective personal pronoun and 'them' is an objective personal pronoun.NOTE: The second blank requires a demonstrative relative pronoun to demonstrate 'amenities'.
Option A - 'What' is a relative pronoun, used to indicate/demonstrate a thing/ideas or for asking for any information indirectly. As the sentence is not about asking information, we can't use 'what' here. Hence, option A is incorrect.
Option B - 'Which' is used as a relative pronoun for the things that have been previously mentioned. As the given sentence doesn't have things that have been previously mentioned, we can't use 'which' here.Hence, option B is incorrect.
Option D - 'His' is used as an objective personal pronoun for 'he'. 'His' is used to indicate any possession or ownership of a male noun/pronoun. Hence, option D is incorrect.
Option C - 'That' is a relative pronoun which is used to demonstrate people or things that aren't close/near to a hand. As the sentence just talks about the amenities (amenities are not near to us), we can use 'that' here. Hence, option C is the correct answer.
'It is small but it has all the amenities that a man can dream of.'