The correct option is
D discussion
In order to fill in the blank with the most suitable noun or verb, let's understand what nouns and verbs are.Nouns are the naming words. For example: Roshni is pretty. Here, 'Roshni' is the noun.
Verbs are the action words. For example: Ram fought bravely. Here, 'Ram' is the noun, and 'fought' is the verb.
NOTE: 'Discuss' can be used in the form of a noun as well as a verb.
Option B - 'Discussing' is the participle form of the verb. Though 'discuss' is a verb, the adjective 'serious' explains to us that we need the noun form of the word 'discuss'.Hence, option B is incorrect.
Option C - 'Discuss' is a verb and not a noun. Thus option C is incorrect.
Option D - 'Discussed' is a verb and not a noun. Hence, option D is incorrect.
Option A - 'Discussion' is a noun and 'serious' explains the 'discussion' as an adjective. Hence, option A is the correct answer.
'Janet and Johnson had a serious discussion over this matter.'