The correct option is
B Their
Option B: 'Their' refers to the people who took part in the games and feats of skill.
The meaning is complete with this word. Hence option B is correct.
Option A: 'The' makes the other words that follow 'skill and alertness' sound unconnected and impersonal with respect to the rest of the sentence.
Option C: 'His' refers to a single person whereas the noun in reference is a group of more than one 'the people'.
Option D: 'Whose' is a relative pronoun that comes before a clause. Here, the words that follow do not form a clause.
Hence options A, C and D are incorrect.