To solve this chemical reaction question, we need to determine the identity of compounds [A] and [B]. Let's break down the reaction step-by-step:

1. The starting compound is a cyclic amine with a side chain, as shown in the given image.
2. The reaction involves two steps:
   1. Step 1: Heating with Zn-dust (Δ) is typically a reduction process. However, in this context, it is likely being used to generate an alkene by dehydrogenation.
   2. Step 2: Oxidation with KMnO₄. This is a common step used to oxidize alkenes to diols or, under more vigorous conditions, to carboxylic acids.
3. Step-by-step identification:
   • In Step 1, dehydrogenation of the cyclic amine's side chain occurs, creating an alkene (compound [A]).
   • In Step 2, the alkene [A] is oxidized by KMnO₄ to produce a carboxylic acid (compound [B]).

According to the given options and the chemical transformations involved, compound [B] is a carboxylic acid derived from the original cyclic amine structure.

Therefore, the correct answer is the third option, represented by the image above.