Which of the following reagents is used to distinguish ethers from...

Ethers contain an oxygen atom with two lone pairs of electrons, which makes them behave as Lewis bases.

When treated with cold concentrated $\text{H}_2\text{SO}_4$, ethers accept a proton from the acid to form oxonium salts, which are soluble in the acid mixture.

$\text{R-O-R} + \text{H}_2\text{SO}_4 \rightarrow [\text{R-OH-R}]^{+} \text{HSO}_4^{-}$

Hydrocarbons (such as alkanes) lack such lone pairs of electrons, do not get protonated, and remain insoluble in cold concentrated $\text{H}_2\text{SO}_4$.

Thus, cold concentrated $\text{H}_2\text{SO}_4$ is used to distinguish ethers from hydrocarbons.

Answer: Cold concentrated $\text{H}_2\text{SO}_4$