Consider the C-H acidity of I cyclopenta-1,3-diene, II ethyne, III propene (most acidic H) and IV propane (most acidic H). The correct order of decreasing acidity is:

Cyclopentadiene gives the aromatic cyclopentadienyl anion (pKa about 16). Ethyne gives an sp carbanion (about 25). Propene gives the resonance-stabilised allyl anion (about 43), and propane gives a simple alkyl carbanion (about 50). Hence I > II > III > IV.