In the following sequence of reactions, ${\text{CH}}_3{\text{CH}}_2\text{OH}\stackrel{{\text{P+I}}_2}{\to }A\stackrel{\text{Mg/ether}}{\to }B\stackrel{\text{HCHO}}{\to }C\stackrel{{\text{H}}_2\text{O}}{\to }D$, the compound 'D' is:

Step 1: ${\text{CH}}_3{\text{CH}}_2\text{OH}\stackrel{{\text{P+I}}_2}{\to }{\text{CH}}_3{\text{CH}}_2\text{I}$ ($A$, Ethyl iodide).Step 2: ${\text{CH}}_3{\text{CH}}_2\text{I}\stackrel{\text{Mg/ether}}{\to }{\text{CH}}_3{\text{CH}}_2\text{MgI}$ ($B$, Ethyl magnesium iodide).Step 3: Grignard reagent adds to formaldehyde ($\text{HCHO}$):
$${\text{CH}}_3{\text{CH}}_2\text{MgI}+\text{H-C(=O)-H}\to {\text{CH}}_3{\text{CH}}_2{\text{CH}}_2\text{OMgI}(C)$$
Step 4: Hydrolysis:
$${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2\text{OMgI}\stackrel{{\text{H}}_2\text{O}}{\to }{\text{CH}}_3{\text{CH}}_2{\text{CH}}_2\text{OH}+\text{Mg(OH)I}$$
Compound $D$ is $1\text{-propanol}$ (n-propyl alcohol).