The correct answer is C. Ground.

1. Ground: This option is correct because it is the past participle form of the verb "grind," which is required after the modal verb "should have" in the perfect aspect. The sentence needs the past participle to form the perfect infinitive structure ("should have ground").

2. Grind: This option is incorrect because it is the base form of the verb, which does not fit the structure needed after "should have."

3. Grinded: This option is incorrect because "grinded" is not the correct past participle form of "grind." The correct past participle is "ground."

4. Grounded: This option is incorrect because "grounded" is the past participle of the verb "ground" in a different context, usually related to being restricted or grounded (e.g., a child grounded from activities). It does not fit the context of grinding pepper.

Summary:

- C. Ground is the correct choice because it is the appropriate past participle form of the verb "grind" used after the modal verb "should have" to indicate an action that was expected to be completed.