It is relatively easy to spot a gerund, given that it looks like a verb + ing. Having that in mind, the gerund here is drinking from the tap. This gerund acts as the subject of the sentence, which you can easily determine by asking the question What is perfectly safe?, and the answer will be Drinking from the tap - that's how you know it is the subject.
So, the correct answer is drinking from the tap/subject.
We need more details, please! I will edit my post with the answer once more information is added.
But words is like the spots on dice: no matter how you fumbles 'em there's times when they just won't come. So simile is the answer.
