How would you describe the difference between gerunds and verbs in a continuous tense to an inquisitive student?

This student may be confused about the difference between gerunds and verbs in a continuous sense (also known as participles) because they look the same - both of these forms have a verb + -ing as their suffix, which means they have the same form.
However, the most important difference is that a gerund is basically a noun - even though it has the same form as a verb, in a sentence it would behave as a noun.
For example: Swimming is fun. (the word swimming is a gerund)
He is swimming at the moment. (the word swimming is a verb/participle)

Hopefully now you can see the difference - it has to do with their grammatical function in a sentence. 

I’d go with letter d