Answer: b.

Explanation:
In this sentence, that isn’t required, so it should be omitted.

From the New York Times, ‘A bit about that.’

Here’s the relevant entry in The Times’s stylebook:

that (conj.). After a verb like said, disclosed or announced, it is often possible to omit that for conciseness: He said he felt peaked. But if the words after said or any other verb can be mistaken for its direct object, the reader may be momentarily led down a false trail, and that must be retained: The mayor disclosed that her plan for the rhubarb festival would cost $3 million.

When a time element follows the verb, that is always needed to make quickly clear whether the time element applies to the material before or after it: The governor announced yesterday that he would organize a knackwurst fiesta.

Often a sentence with two parallel clauses requires the expression and that in the second part; in such a case, keep that in the first part also, for balance: The mayor said that she might run again and that if she did, her brother would be her campaign manager.

Follow us on Twitter @tao_of_grammar