$\mathrm{RMgX}+{\mathrm{CO}}_2\underset{\mathrm{ether}}{\overset{\mathrm{dry}}{\to }}\mathrm{Y}\stackrel{{\mathrm{H}}_3{\mathrm{O}}^{+}}{⟶}\mathrm{RCOOH}$

What is $\mathrm{Y}$ in the above reaction?

Grignard reagent on reaction with carbondioxide in presence of dry ether forms an addition product which on hydrolysis produces carboxylic acid.

 Hence,  $\mathrm{Y}$ in the above reaction is ${\mathrm{RCOO}}^{-}{\mathrm{Mg}}^{+}\mathrm{X}$