Verbs of opinion and belief (penser que, croire que, être sûr que) take the indicative, not the subjunctive, when used affirmatively, because they're asserting something as a fact: je pense qu'il a raison (I think he's right) uses the indicative (a). Only when these same verbs are negated or questioned — and the certainty disappears — does the subjunctive take over: je ne pense pas qu'il ait raison (I don't think he's right).
This flip is one of the more subtle points of French grammar: the exact same verb, penser, triggers the indicative when affirmative and the subjunctive when negative, purely because negating an opinion turns a stated fact into a doubted one. Some speakers are more lenient about this in casual conversation, but it remains the standard rule in formal and written French.