Since fishing fleets might not have been so inefficient in limiting their catch to target species _____ more strict in enforcing penalties.

Had the government been

The meaning to this sentence might no be too accessible to everyone, so you break it up into segments and look for syntax sequence. It starts with the connector SINCE, the verb is HAVE NOT BEEN. You should identify that after the blank there is no verb. On that premise you eliminate option D. You can also eliminate option B because it only has HAD and you need a BE form to complement the first verb. After this, you should remember your TOEFL skills on subject / verb inversion for this kind of sentence. Option A is eliminated. Leaving option C as the correct option.