What type of reaction is the formation of aldol from aldehyde?

Identify the product $Y$ in the following reaction: $CH_3-CH=CH-CH_2-CHO + H_2 \xrightarrow{Ni} Y$

The synthesis of crotonaldehyde from acetaldehyde is an example of ....... reaction.

Benzaldehyde on heating with acetone in the presence of $NaOH$ gives:

Identify the products of the following reaction: $HCHO + C_6H_5CHO \xrightarrow[ii) H_3O^{+}]{i) conc. NaOH} \text{Products}$

What are $X$ and $Y$ respectively in the following reactions?
$C_6H_5COOH \xrightarrow{X} C_6H_5CHO$
$C_6H_5CHO \xrightarrow[OH^-, 293 \ K]{C_6H_5COOH} Y \text{ (Major product)}$