Which of the following compounds has the highest $K_a$ value?

Write the structure of the product of the following reaction.

Identify the major product formed when the following compound is treated with acidic $KMnO_4$.

Arrange the following compounds in increasing order of their $pK_{a}$ values:
$(a)$ $p$-nitrobenzoic acid
$(b)$ $p$-methoxybenzoic acid
$(c)$ $p$-nitrophenol
$(d)$ benzoic acid

When carboxylic acid reacts with organolithium reagents to give ketones,side reactions sometimes occur. For example,
$HOCH_2CH_2CH(CH_3)CH_2CH_2COOH$ $\xrightarrow[(x) \, CH_3Li]{tetrahydro\,furan}$ $\xrightarrow[NH_4Cl]{H_2O}$ $\underset{63\%}{\text{Compound } A: HOCH_2CH_2CH(CH_3)CH_2CH_2COCH_3}$ + $\underset{37\%}{\text{Compound } (B)}$
The value of $(x)$ in the above reaction is:

The correct order of acid strengths of benzoic acid $(X)$,peroxybenzoic acid $(Y)$ and $p-$nitrobenzoic acid $(Z)$ is