The reagent, from the following, which converts benzoic acid to benzaldehyde in one step is

To convert benzoic acid directly into benzaldehyde, the reagent must reduce the carboxylic acid only up to the aldehyde stage.

$$\mathrm{LiAlH_4}$$ is a strong reducing agent and over-reduces benzoic acid to:

$$\mathrm{Benzyl\ Alcohol}$$

Hence, it is unsuitable.

$$\mathrm{NaBH_4}$$ is a mild reducing agent and does not reduce carboxylic acids.

$$\mathrm{KMnO_4}$$ is a strong oxidising agent and cannot perform this reduction.

$$\mathrm{MnO}$$ in presence of formic acid converts benzoic acid into benzaldehyde through catalytic pyrolytic decarboxylation.

Therefore, the correct reagent is:

$$\mathrm{MnO}$$

Correct Option: $$\mathrm{MnO}$$. Thus, the right option is C.