Fill in the blanks with the most appropriate option:-

Once you learn …………. you will never forget it.

how to swim

The correct option to fill in the blank is how to swim.

The phrase "how to swim" is the most common way to say "the ability to swim". It is the most appropriate word to use in this context, as the sentence is about learning something that you will never forget.

The other options are not as appropriate:

• To swim is a gerund, which is a verb form that is used as a noun. It would not be accurate to use a gerund in this context, as the sentence is about learning something.
• To swimming is not a grammatically correct phrase. It is a combination of the gerund "swimming" and the preposition "to". It would not be accurate to use this phrase in this context.
• Swim is the infinitive form of the verb "to swim". It would not be accurate to use the infinitive form in this context, as the sentence is about learning something.