'Grind' is the base form of the verb, its past and past participle form is 'ground'. Grind is an irregular verb as its past and its past participle form is not formed with the addition of -ed at the end of the base form of the verb.
Therefore, option A 'grind' and option B 'ground' are incorrect, whereas, grinding is the continuous form of the verb 'grind', therefore option C is incorrect.
The correct option is option D, 'guard' as its past and past participle form is 'guarded'. It's a regular verb because we've simply added an -ed to form the past and past participle form of the base verb 'guard'.