1. Identify the transformation

We need to convert:

$C_2H_5CN \;\to\; CH_3CH_2CH_2NH_2$

Here,

• $C_2H_5CN = CH_3CH_2C\equiv N$ (a nitrile, propanenitrile)
• Product $CH_3CH_2CH_2NH_2$ is propylamine

So this is the reduction of a nitrile to a primary amine.

In general:

$R-C\equiv N \xrightarrow[\text{hydrolysis/workup}]{} R-CH_2NH_2$

2. Check the suitable reducing agent

A nitrile requires a strong reducing agent for conversion into a primary amine. The standard reagent is:

$LiAlH_4$

Reaction:

$R-C\equiv N \xrightarrow{LiAlH_4} R-CH_2NH_2$

Thus,

$CH_3CH_2C\equiv N \xrightarrow{LiAlH_4} CH_3CH_2CH_2NH_2$

3. Evaluate each option

• A: $NaBH_4$
  This is a mild reducing agent. It reduces aldehydes and ketones, but does not normally reduce nitriles.
  Hence, incorrect.

• B: $CaH_2$
  Calcium hydride is mainly used as a drying agent/base, not for reducing nitriles to amines.
  Hence, incorrect.

• C: $Na(CN)BH_3$
  Sodium cyanoborohydride is a mild reducing agent used mainly in reductive amination. It is not suitable for direct reduction of nitriles.
  Hence, incorrect.

• D: $LiAlH_4$
  Strong reducing agent; reduces nitriles to primary amines.
  Hence, correct.

4. Conclusion

The most appropriate reagent is:

$\boxed{LiAlH_4}$

So the correct option is D.