The most suitable substrates for preparation of following alcohol is

The correct answer is Option (3) →

Grignard reagent reaction:

Formaldehyde → Primary alcohol

$R-MgBr+HCHO→ R-CH_2OH$

Detailed Explanation

Target alcohol:

$(CH_3)_2CH-CH_2OH$

Primary alcohol with one extra carbon added via formaldehyde.

Option 1: $\text{CH}_3\text{CHO} + \text{CH}_3\text{CH}_2\text{MgBr}$

Aldehyde gives secondary alcohol.

Option 2: $\text{CH}_3\text{COCH}_3 + \text{CH}_3\text{MgBr}$

Ketone gives tertiary alcohol.

Option 3: $\text{CH}_2\text{O} + (\text{CH}_3)_2\text{CHMgBr}$

Formaldehyde gives primary alcohol:

$(\text{CH}_3)_2\text{CH—CH}_2\text{OH}$

Option 4: $\text{CH}_3\text{CH}_2\text{CHO} + \text{CH}_3\text{MgBr}$

Aldehyde gives secondary alcohol.