$HCHO + CH_3CHO$ $\xrightarrow[Heat]{dil. NaOH} A$ $\xrightarrow{HCN, H_3O^+} B$. What is the formula of $B$ in this reaction?

The end product $(C)$ of the reaction is:

Acetophenone,when reacted with a base,$C_2H_5ONa$,yields a stable compound which has the structure:

Aldehydes and ketones can be reduced to corresponding hydrocarbons by

Given below are two statements $:$
Statement $(I) :$ Vanillin, which has a benzene ring substituted with a methoxy group, a hydroxyl group, and an aldehyde group, will react with $NaOH$ and also with Tollen's reagent.
Statement $(II) :$ Vanillin will undergo self-aldol condensation very easily.
In the light of the above statements, choose the most appropriate answer from the options given below $:$

Which of the following reagent$(s)$ is/are used for the given conversion?