Option D is correct
Both the compound s are aldehyde showing reducing property. So they ca not be distinguished by Fehling's solutiin or Tolkien's reagent. Again they cannot be distinguished by 2,4DNP solution which detects the presence carbonyl group.Here both compounds respond to this reagent.
Reagents used inD detect the presence of keto-methylje group present in carbonyl compound. Here actaldhydeac responds to this test but formaldhyde does not.
