To solve this problem, we need to determine the structure of alkene $A$ and the major product $B. \\$Step 1: Identify the alkene $A. \\$The ozonolysis of alkene $A$ gives propanone and ethanal in equimolar ratio.$\\$The structure of propanone is $$CH$_3$COCH$_3$ and ethanal is $$CH$_3$CHO$. \\$This suggests that the alkene $A$ is 2-methylbut-2-ene:$\\$CH$_3$C(CH$_3$)=CHCH$_3. \\$Step 2: Determine the major product $B$ from the addition of $HCl. \\$The addition of $HCl$ to an alkene follows Markovnikov's rule, where the hydrogen adds to the carbon with more hydrogens.$\\$For 2-methylbut-2-ene:$\\$CH$_3$C(CH$_3$)=CHCH$_3 +$HCl$\rightarrow$CH$_3$C(CH$_3$)(Cl)CH$_2$CH$_3. \\$Step 3: Identify the structure of product $B. \\$The major product $B$ is 2-chloro-2-methylbutane.$\\$Comparing with the options, Option 3 matches this structure:$\\$CH$_3$C(CH$_3$)(Cl)CH$_2$CH$_3. \\$Therefore, the correct option is Option 3.