The correct option is
C Them
Pronouns replace nouns that have been mentioned before.
The given sentence is about trees. The noun 'trees' has been mentioned before and would be replaced by a pronoun to avoid repetition and monotony.
Option C: 'Them' is a plural pronoun that refers to objects or people. Hence it replaces the noun 'trees' and option C is correct.
Option A: 'Him' refers to a man. 'Trees' is not a man.
Option B: 'It' refers to one single object. The noun 'trees' is plural.
Option D: 'Their' is a possessive pronoun. The given sentence does not express possession of anything.
Hence options A, B and D are incorrect.