To identify the final product 'D' in a multi-step organic reaction sequence, one must sequentially analyze the action of each reagent on the substrate:

1. Identify Functional Groups: Determine the reactive centers in the starting material (e.g., double bonds, halides, carbonyls).
2. Reagent Function: Apply the specific transformation associated with each reagent (e.g., Grignard reagents for nucleophilic addition, $KMnO_4$ for oxidation, $LiAlH_4$ for reduction).
3. Stereochemistry & Regiochemistry: Consider Markovnikov/Anti-Markovnikov rules for additions or inversion of configuration ($S_N2$) where applicable.
4. Sequential Transformation: Track the structure from Reactant $\rightarrow$ A $\rightarrow$ B $\rightarrow$ C $\rightarrow$ D.

Common sequences in Class 12 Organic Chemistry include converting alkyl halides to amines (Unit 13), alcohols to carboxylic acids (Unit 11, 12), or utilizing diazonium salts for aromatic substitutions.