Which of the following exhibit intermolecular hydrogen bonding in the pure liquid state?

A pure substance must contain both an H bonded to F, O or N (donor) and a lone-pair-bearing acceptor atom. $\mathrm{NH}{}_3$ and $\mathrm{CH}{}_3\mathrm{COOH}$ have N-H and O-H bonds respectively. Dimethyl ether has no O-H and methane has no polar H, so they cannot hydrogen bond with themselves.