Best reagent to convert 4-chloro-4H-pyran into an aromatic compound
In order to choose the best reagent to convert the compound to an aromatic one, let's ask the question of what is preventing this compound from being aromatic? At carbon-4, the pi-system is disrupted by an sp3-hybridized atom. If carbon-4 could be converted to sp2 by some reaction, then the compound could become aromatic. At C-4, we see a chloride. We know that chlorides are good leaving groups, so let's look at the product of that step.



After chloride leaves, there's a cyclic, fully conjugated pi-system containing six electrons (two for each double bond, two from the oxygen, and zero from the carbocation). Drawing a resonance structure shows that this is an oxygen analog of benzene (known as pyrylium ion).

What kind of reagent would promote this process, i.e., a halide leaving? A Lewis acid, such as $\ce{AlCl3}$ could promote this in two ways. First, it could interact with the chloride in the starting material, building up positive charge on the chloride, making it a better leaving group. Secondly, the Lewis acid can coordinate the chloride on the product side, preventing it from attacking the pyrylium ion.