Probability - Mutually exclusive event prepswift quiz


In this question, How can it be assumed that event a and b are the only 2 possible events.
I assumed a dice, where event a can be rolling a one, and event b can be rolling a 2.
there the maximum possible value is less than 1.
Considering a dice and a coin case I came to the ans choice D, but the correct ans is C
Please help out!

Hint: “maximum”