Question

Tiny particles expelled from the comet’s core make up the dust’s _____ of the comet, which is several million kilometers long.what's the answer to fill in the blank?

Answer 1

The right word is TAIL.

This paragraph referers to the dust tail of a comet, which can be seen in the next image:

It can reach tens of millions of kilometers and it is composed of dust particles with a diameter of about a micrometre. Typically it is white or yellow in colour.