An ATP molecule is structurally most similar to a molecule of

• RNA consists of ribose nucleotides (nitrogenous bases appended to a ribose sugar) attached by phosphodiester bonds, forming strands of varying lengths. 
  
• ATP is a type of ribose nucleotide, having an adenine base, a ribose sugar, and three phosphate groups. 
• ATP is the energy currency of our cells. 

So, the correct answer is 'RNA molecule'