Final answer:
The opposite of 'lull' is 'upset' since it suggests introducing disturbance to a period of calm. 'Dislike', 'fool', and 'mull over' do not represent this type of disturbance and are hence incorrect as antonyms for 'lull'.
Step-by-step explanation:
The word lull typically means a temporary period of quiet or calm, often suggesting a contrast to previous or subsequent activity or noise. The antonyms of lull would logically be words that suggest activity, noise, disturbance, or agitation. Given the provided options, the most suitable opposite of lull would be upset, as to upset something parallels introducing disturbance or breaking the calm, much in the same way a lull signifies a break in disturbance.
Option (b) dislike is unrelated because it signifies a feeling rather than an action or state of quiet. Similarly, (c) fool is incongruent since it connotes deception or trickery, not necessarily a disturbance or activity. Lastly, (d) mull over implies thinking something over carefully, which does not inherently disrupt a state of calm or quiet.
The correct answer to the question is (a) upset.