What is Tollen's reagent?

Identify the reaction in which the carbonyl group of aldehydes and ketones is reduced to a methylene group upon treatment with zinc-amalgam and concentrated hydrochloric acid.

Which carbonyl group is most reactive for $NAR$ (Nucleophilic Addition Reaction)?

The reactant $(A)$ will be:

Which among the following reagents cannot be used for the above interconversion? The reaction involves the reduction of a ketone group $(>C=O)$ to a methylene group $(-CH_2-)$.

Identify the reagents $P$ and $Q$ in the following reaction sequence: