I would eliminate D more for the construction of 'of human cell's different genes' -- the 50,000 to 100,000 is modifying genes, so it is clearly to have them closer together.

Also, the article 'the' is needed before the numbers

As far as your question:

The difference is in the type of speech.

Merely is an adverb, which typically modifies adjective, verbs, or other adverbs (They can modify nouns in rare circumstances, such as gerunds, but without digressing too far, gerunds take on verb characteristics in sentence structure)

Mere is an adjective, which will modify a noun.


Now, I could argue that merely is modifying three (an adjective) or that three percent is a compound noun. To me a bit tricky to say with certainty, so I revert to the earlier reasoning.