The correct answer is C. It changes the verb infect to an adjective, infectious.
Here's why:
- "Infect" is primarily a verb, meaning to taint or contaminate.
- Adding the suffix "-ous" transforms "infect" into "infectious," which describes something that can spread or is capable of causing infection.
- Therefore, by adding "-ous," we are converting the verb "infect" into an adjective "infectious," which modifies nouns by describing a characteristic (in this case, the ability to spread infection).