Explanation:

The original sentence is erroneous.

Reason: The correct idiomatic expression is 'a helping hand' and not ‘a helpful handshake'.

Lend a helping hand (Idiom):

Meaning: To help or assist.

E.g.: Peter is always willing to lend a helping hand around the house.

Hence ‘a helping hand’ should be used in place of ‘a helpful handshake’ to make the sentence grammatically correct.

Among the given choices, only option C replaces the given bold part most appropriately.

The sentence after replacement becomes:

India lent a helping hand to Nepal by giving them 2.1 billion Nepalese rupees for the reconstruction of houses flattened in the 2015 earthquake.

Option C is hence the correct answer.