Which of the following refers to pebble-sized bits of pyroclastic material that range in diameter from 2 to 64 mm?

The term for pebble-sized bits of pyroclastic material that range in diameter from 2 to 64 mm is lapilli. This classification of volcanic material specifically describes the fragment size; lapilli are larger than ash and smaller than volcanic blocks or bombs.

Tephra is a broader term that encompasses all types of volcanic debris ejected during an eruption, including ash, lapilli, and larger fragments. Cinders typically refer to smaller volcanic particles, often less than 2 mm in diameter, and are associated with specific types of volcanic eruptions. Pumice, on the other hand, refers to a highly porous volcanic rock formed when lava cools rapidly and traps gas bubbles. While pumice may sometimes appear in similar contexts of volcanic activity, it is not classified by particle size in the same way as lapilli.

Thus, lapilli is the most accurate term for the size range specified in the question.