Explanation

(c)
Carboxylic acids are prepared by reaction of Grignard reagent with $CO_{2}$.
$\because$ Formic acid $(HCOOH)has\ only\ one\ carbon\ atom$
$\therefore$ Formic acid cannot be prepared from Grignard reagent.
$O$
$||$
$RMgX + CO_{2} \rightarrow R - C - OMgX\overset{HOH\ \ \ }{\rightarrow}RCOOH$
Grignard reagent acid