Final answer:
The correct word to complete the sentence about the event planner's requirements for Tuesday's forum is 'additional', as it serves as an adjective that correctly describes the needed chairs.
Step-by-step explanation:
The correct answer to fill in the blank in the given sentence is 'c) additional'. The sentence should read: The event planner determined that Tuesday’s forum will require additional chairs. This is the correct choice because 'additional' serves as an adjective describing the noun 'chairs', which implies that more chairs are needed beyond what is already available. Option 'a)' is incorrect because 'addition' is a noun, and the sentence requires an adjective. Option 'b)' 'additions' is also a noun and doesn't fit grammatically in the sentence. Lastly, option 'd)' 'additionally' is an adverb and it does not suit the structure of the sentence where an adjective is required.