Consider the above reaction and identify "Y"

DIBAL-H (diisobutylaluminium hydride) is a selective reducing agent that converts nitriles into aldehydes under controlled reaction conditions.

In the first step, the nitrile $$\mathrm{R-CN}$$ is partially reduced by DIBAL-H to form an imine intermediate.

Upon subsequent hydrolysis with water, the imine is converted into the corresponding aldehyde.

The overall transformation is

$$\mathrm{R-CN \xrightarrow[\text{2. }H_2O]{\text{1. DIBAL-H}} R-CHO}$$

Thus, the functional group $$Y$$ is

$$\boxed{\mathrm{-CHO}}$$

Hence, the correct answer is

$$\boxed{\text{Option (C)}}$$