The correct answer is: A. Mary said to Robert that if he came, they would see him.
The sentence “Let him come, then we shall see” is a conditional sentence. The first clause, “Let him come,” is the condition, and the second clause, “then we shall see,” is the result. The sentence means that if Robert comes, Mary and Robert will see him.
Option B is incorrect because it uses the word “shall,” which is a modal verb that is used to express obligation or necessity. In this sentence, “shall” does not make sense because there is no obligation for Robert to come.
Option C is incorrect because it uses the word “once,” which indicates a single event. In this sentence, “once” does not make sense because there is no indication that Robert will only come once.
Option D is incorrect because it uses the word “might,” which indicates possibility. In this sentence, “might” does not make sense because there is no indication that Robert might come.