The correct option is
C success
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: 'Success' can be used in the form of a noun as well as a verb.
Option A - 'Succeeded' can't be used here, as the adjective 'great' explains the noun form of 'succeed'. Hence, option A is incorrect.
Option B - 'Successful' is an adjective and not a noun. Hence, option B is incorrect.
Option D - 'Successfully' is an adjective and not a noun. Hence, option D is incorrect.
Option C - 'Success' is a noun, which is described in the sentence by the adjective 'great'. Hence, option C is the correct answer.
'This project has been a great success.'