Subject-verb agreement with collective nouns is the key idea here. When a group like "the committee" is treated as a single unit, the verb should be singular. The sentence uses the singular auxiliary has with the past participle decided, signaling that the group as a unit has made a decision. That alignment—singular verb with a singular collective noun—fits standard usage and clearly expresses a completed action by the group. The other sentences mix number or aspect in ways that don’t align with treating the committee as one unit: using have or were implies the group is acting as individuals, which isn’t the typical American standard for a unit; and is deciding suggests ongoing action rather than a finished decision.