Read the sentence.
the comets shed the ice that forms their glowing tails.
which subject agrees with the verb forms?
a. tails
b. that
c. ice
d. comets

The answer is C: ice.

 The word ice is part of the subject the ice, which agrees with the verb forms. The subject is simple and requires the verb in the third person of singular, of the simple present tense (to form - it forms). The subject comets agrees with the verb shed (simple past tense). The word tails is part of the direct object their glowing tails. The word that is a determiner.

b. b. b. is the anoswer