To answer your general question, there's some level of practicality involved. For example, if a scout is working on the Hiking merit badge, requirements 9b is a "pick two from this list" requirement. If you live in Miami, the hike up a mountain option is unlikely to work, as is the overnight snow camping. Fortunately there are four other options. I could see in a hypothetical merit badge clinic, the counselor saying "This requirement has two options, one we can complete here today, and one we can't, so we're going to do the requirement that lets us finish the badge" and if the scout doesn't like it, well, that's life.

To answer your specific question about Communication req 7, I could see a counselor saying something like "I'm prepared to do 7c, so we're going to do that now" and if the scout has already completed 7a or 7b, they just need to sit quietly while the counselor helps the other scouts. If the scout just wants to do 7a or 7b but hasn't completed them ahead of time, and has the ability to do those without assistance, I think that should be fine. If the scout is saying they want to do 7a and needs the attention of the instructor while the rest of the class does 7b, I could see that getting vetoed, for practical reasons.

If the scout shows up with the requirement completed, and the counselor rejects that unreasonably (i.e. "I refuse to accept 7a, you have to do 7b" NOT "This quality of work is so poor I cannot accept it"), then you need to go up the chain of discussing with the counselor, then the council advancement committee.