Tautomers are isomers that:

Tautomers are isomers that differ in the placement of functional groups, while having the same molecular formula. Option A is incorrect because tautomers have the same molecular formula, but differ in their functional groups. Option C is incorrect because tautomers can differ in more than just the position of a hydrogen atom. Option D is incorrect because tautomers are not mirror images, rather, they have different functional groups. Option B is the correct answer because tautomers have the same atoms, but the arrangement of their double or single bonds differs.