In the following sequence of reactions (shown in the figure), the compound D is

Ethanol + P/$\mathrm{I_2}$ gives ethyl iodide (A); Mg/ether gives ethylmagnesium iodide (B); addition to formaldehyde gives $\mathrm{CH_3CH_2CH_2OMgI}$ (C); hydrolysis gives propan-1-ol (D). A Grignard reagent plus HCHO always gives a primary alcohol with one more carbon.