You memorize a series of numbers by grouping them into sets of three and thinking of these sets as area codes. this is an example of:

The correct answer is chunking.

Explanation:

Chunking is a term signifying the procedure of taking single pieces of information or chunks and aligning them into bigger units. By aligning each piece into a large whole, one can better the amount of data one can remember.  

Generally, the most common illustration of chunking takes place in phone numbers. By distinguishing dissimilar single elements into bigger blocks, information becomes easier to recall and retain.