The four types of subject discussed in this letter are singular, plural, compound, and

The four types of subject discussed in this letter are singular, plural, compound, and collective.
Collective pronouns are used to denote groups of nouns, and words such as 'team, police, class,' etc. belong in this group. The remaining options are gerund (nouns which look like verbs and end in -ing), phrase, and preposition nouns, neither one of which exists. 

Inform should be the answer

What was your independent reading novel