Step-by-step Reaction Analysis: 1.Reaction of C$_2$H$_5$Br with alcoholic KOH: C$_2$H$_5$Br (ethyl bromide) undergoes dehydrohalogenation with alcoholic KOH to form ethene (CH$_2$ = CH$_2$). This step is represented as: \[\text{C}_2\text{H}_5\text{Br} \xrightarrow{\text{alc. KOH}} \text{CH}_2 = \text{CH}_2 \; (\text{A})\] 2. Addition of Br$_2$ in CCl$_4$: Ethene reacts with bromine (Br$_2$) in the presence of carbon tetrachloride (CCl$_4$) to form 1,2-dibromoethane (BrCH$_2$CH$_2$Br). This step is represented as: \[\text{CH}_2 = \text{CH}_2 \xrightarrow{\text{Br}_2/\text{CCl}_4} \text{BrCH}_2\text{CH}_2\text{Br} \; (\text{B})\] 3. Reaction of 1,2-dibromoethane with excess KCN: The compound BrCH$_2$CH$_2$Br reacts with excess potassium cyanide (KCN) to form 1,2-dicyanoethane (NCCH$_2$CH$_2$CN). This step is represented as: \[\text{BrCH}_2\text{CH}_2\text{Br} \xrightarrow{\text{Excess KCN}} \text{NCCH}_2\text{CH}_2\text{CN} \; (\text{C})\] 4. Hydrolysis of 1,2-dicyanoethane: Hydrolysis of 1,2-dicyanoethane (NCCH$_2$CH$_2$CN) in the presence of excess H$_3$O$^+$ (acidic medium) yields succinic acid (HOOCCH$_2$CH$_2$COOH). This step is represented as: \[\text{NCCH}_2\text{CH}_2\text{CN} \xrightarrow{\text{H}_3\text{O}^+\text{ (Excess)}} \text{HOOCCH}_2\text{CH}_2\text{COOH} \; (\text{D})\] Conclusion: The acid D formed is succinic acid.
