The only way to convert between two substances is to use a to conversion. gram to gram gram to mole mole to molecule mole to mole

Mole to Mole.

Assuming you're referring to converting in general, there must always be a mole to mole conversion.

Gram to gram would be incorrect as you must have a molar conversion in order to do so.

Gram to mole would be incorrect because that would leave out molecular conversions which would require three steps rather than two.

Mole to molecule is incorrect as it leaves out grams, depending on the circumstances and equation provided.

Thus, through process of elimination, mole to mole is the correct answer.

Hope this helps!