By a proper choice of reagents, both symmetrical and unsymmetrical ethers can be prepared by Williamson synthesis which involves the reaction between an alkyl halide and an alkoxide ion. The reverse process involving the cleavage of ethers to give back the original alkyl halide and the alcohol can be carried out by heating the ether with HI at 373K.

Which of the following ethers is not cleaved by HI? 

Diphenyl ether

The correct answer is option 4. Diphenyl ether.

Diphenyl ether is not easily cleaved by hydroiodic acid (HI) due to the stability imparted by the phenyl groups attached to the ether oxygen. The benzene rings in diphenyl ether stabilize any carbocation that would form during the cleavage process through resonance stabilization.

In contrast, ethers with alkyl groups (such as dicyclohexyl ether or di-tert-butyl ether) generally have less stabilization of carbocations, making them more susceptible to cleavage by HI. Therefore, diphenyl ether (\(\text{C}_6\text{H}_5\text{O}\text{C}_6\text{H}_5\)) is not cleaved by HI, making option 4 the correct answer in this context.