The highlighted word means 'sleepy'.
Option A is correct. 'Sluggish', in this context, also means 'sleepy'. Thus, it has the required meaning and is the correct answer.
Options B, C and D are incorrect. They do not have the required meaning.
'Irritable' means 'easily annoyed', 'energetic' means 'active' and 'dizzy' means 'woozy' or 'sick'.