Final answer:
Tephra that is 1 mm in size is referred to as volcanic ash, which is composed of rock, mineral, and glass fragments from a volcanic eruption.
Step-by-step explanation:
When studying volcanic ash, it is critical to understand the various sizes of tephra, which are fragments of rock, minerals, and volcanic glass, produced during volcanic eruptions. Tephra that measures less than 2 mm in diameter is commonly referred to as volcanic ash. Therefore, the correct answer to what tephra that is 1 mm in size is called, is (d) ash.