Final answer:
The correct word to complete the sentence is 'additional,' as it serves as an adjective describing the chairs that are required for Tuesday’s forum.
Step-by-step explanation:
The correct answer to fill in the blank in the given sentence is 'additional.' The complete sentence would be: 'The event planner determined that Tuesday’s forum will require additional chairs.'
When deciding which form of the word 'addition' to use, it's important to look at the role the word plays in the sentence. In this case, the word is used as an adjective to describe the chairs that are needed. Therefore, 'additional', which is an adjective, fits properly in the sentence to modify the noun 'chairs'.