Which of the following compounds is most stable?

The compounds $(X)$ and $(Y)$ are formed by replacing $H_a$ with $D$ (while keeping $H_b$ as $H$) and replacing $H_b$ with $D$ (while keeping $H_a$ as $H$) respectively in the given molecule. What is the relationship between $(X)$ and $(Y)$?

Give the name and structure of the following:
$(i)$ Chain isomers of pentane $(C_5H_{12})$
$(ii)$ Isomers of $C_3H_8O$
$(iii)$ Isomers of $C_3H_6O$
$(iv)$ Metamers in $C_4H_{10}O$

Which of the following statements is correct about $A$ and $B$?

The major product$(s)$ of the reaction is (are):

Which of the following structures represent the same molecule?