When John was asked to memorize 15 alphabets in a particular order, he organized them into groups of three letters each and memorized them in the given order. In this scenario, John has used the tactic of

Chunking

Explanation:

Chunking is a term referring to the process of taking individual pieces of information (chunks) and grouping them into larger units. By grouping each piece into a large whole, you can improve the amount of information you can remember. So that's what John did by grouping them into 3 letters each.

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