'Sit' is the base form of the verb, its past form is 'sat' and its past participle is 'sat'. Sit is an irregular verb as its past and past participle form is not formed with the addition of -ed at the end of the base form of the verb.
Therefore, option B 'sit' and option C 'sat' are incorrect.
Option A will also be incorrect as 'shut' is an irregular verb as its past and past participle form is not formed with the addition of -ed at the end of the base form of the verb.
The correct option is D, 'seated' as it is the past and past participle form of the base verb 'seat'. It's a regular verb because we've simply added an -ed  to form the past and past participle form of the base verb 'seat'.