When formic acid reacts with \[PC{l_5}\] it forms.
(a) Formyl chloride
(b) Acetyl chloride
(c) Methyl chloride
(d) propionyl chloride

Hint: The acetyl chloride (\[RCOCl\]) can be prepared by chlorination of carboxylic acid (\[RCOOH\]).

Complete step-by-step answer:
The phosphorous penta chloride (\[PC{l_5}\]) is a chlorinating and dehydrating agent. It is a colourless solid and is sensitive to water and moisture.
In synthetic organic chemistry, chlorination can be done in two ways.
Oxidative chlorination-
                   It involves the transfer of chlorine molecules from the reagent to the substrate.
Substitutive chlorination-
                      It involves the replacement of oxygen (\[O\] ) and hydroxyl group (\[OH\] ) of acid.
The \[PC{l_5}\] can be used for both oxidative and substitutive chlorination. And it can convert acid to acyl chloride and alcohols to alkyl chloride.
                                                         \[RCOOH + PC{l_5} \to RCOCl\]
                                                           \[ROH + PC{l_5} \to RCl\]
Similarly, when the \[PC{l_5}\]react with formic acid (\[HCOOH\]) or acetic acid (\[C{H_3}COOH\]) it converted them into formyl chloride(\[HCOCl\]) and acyl chloride (\[RCOCl\]) mostly along with the phosphorous oxychloride (\[POC{l_3}\]) and hydrochloric acid (\[HCl\] ) as a byproduct.
                         \[\] \[HCOOH + PC{l_5} \to HCOCl + POC{l_3} + HCl\]
When the \[PC{l_5}\]react with alcohols such as methyl alcohol (\[C{H_3}OH\] ) and propionyl alcohols (\[C{H_3}C{H_2}OH\]) it forms methyl and propionyl chloride respectively. The byproduct of chlorination of alcohols is the same as the byproduct of acids.
                                                \[C{H_3}OH + PC{l_5} \to C{H_3}Cl + POC{l_3}\]
                                                \[C{H_3}C{H_3}C{H_3}OH + PC{l_5} \to C{H_3}C{H_2}C{H_2}Cl + POC{l_3}\]

The above points indicate that upon chlorination of formic acid with \[PC{l_5}\]the formation of formyl chloride is possible. Therefore, the answer is (a).

Note: Phosphorous penta chloride (\[PC{l_5}\]) reacts with water vigorously with the liberation of \[HCl\] gas.
The \[PC{l_5}\]has a trigonal bipyramidal shape.