Which of the following is the most acidic substance?

The major product formed in the reaction is

Identify the most acidic hydrogen in the given compound.

$C_6H_5CH_2MgBr$ $\xrightarrow[\text{(ii) } H_3O^{+}]{\text{(i) } CO_2 / \text{Ether}} \text{'X'}$ $\xrightarrow[\Delta]{NaOH + CaO} \text{'Y'}$
What is the final product in this reaction?

Arrange these carboxylic acids in decreasing order of ease of esterification with $CH_3OH/H^{+}$:
$(I)$ $CH_3-CH(CH_3)-COOH$
$(II)$ $CH_3-CH(CH_3)-CH_2-COOH$
$(III)$ $CH_3-C(CH_3)_2-COOH$
$(IV)$ $(CH_3-CH_2)_3C-COOH$

In which of the following reactions will cyanide be obtained as the major product?