What is the infinitive of the verb form of this word? example: promotion--to promote salvation

The infinitive form of the verb refers to the basic form of the verb. The infinitive verb preceded by 'to', for example: to run. to eat, to save and so on.

Here 'to' is not a preposition but the sign of infinitive.

In the above word, the infinitive form of the verb of salvation is 'to save'.

Salvation refers to the act of saving someone from any kind of harm, risk or cause.

b is the answer for this question.