Which of the following alkenes is the most stable?

The correct structure of the product $A$ formed in the reaction shown below is:
$Cyclohex-2-en-1-one \xrightarrow[{Pd/C, \text{ ethanol}}]{{H_2 \text{ (gas, 1 atm)}}} A$

What is the major product expected from the following reaction?
Where $D$ is an isotope of hydrogen.

When $2-$Methylbut$-2-$ene is treated with hydrogen chloride,the major product obtained is

Ethyl phenyl acetylene ($1$-phenyl-but-$1$-yne) on reduction with partially deactivated palladised charcoal (Lindlar's catalyst) gives:

Identify the major product $X$ in the following reaction:
$C_6H_5-CH_2-CH=CH_2 \xrightarrow{HBr, R_2O_2} X$