What is the following activity known as in vocabulary learning ?
Find words which can befriend the word ‘rain’. e.g. heavy rain.

The correct answer is Collocation.

Explanation:

Collocation refers to a natural combination or sequence of words that frequently occur together in a language. These word pairings sound natural to native speakers, such as pairing the noun "rain" with the adjective "heavy" to form "heavy rain."

Let's look at why the other options do not fit this specific description:

1. Phrase: A phrase is a group of two or more words functioning as a meaningful unit within a sentence (e.g., "in the garden"), but it does not specifically refer to words that naturally "befriend" or pair with a specific target word.

2. Word groups: This is a general term for sets of words grouped together, often by category or topic, rather than by natural co-occurrence.

3. Word web: A word web is a visual diagram or graphic organizer used to map out vocabulary related to a central topic or theme.

Therefore, finding words that naturally pair or "befriend" a word like "rain" (e.g., "heavy rain", "torrential rain", "pour with rain") is known as Collocation.