The test question is poor. Death may be regarded as an uncountable thing which requires a singular form: death comes to us all, or a countable thing: a pet's death is sad. If this distinction, which, as @James K says, is optional, is made, it is usually between:
(1) death in general or in the abstract
the government deplored 100 people's death in a flood and
(2) the deaths of individuals or types of death
100 people's deaths in a flood were mourned by their relatives
Police recorded 50 people's deaths from stabbing and 65 from car accidents in my town last year
Many people do not make this distinction and simply use singular or plural depending on the number, as you wished to in your answer.
It may be worth noting that, in the context of e.g. a riot, natural disaster, car or plane crash, etc, it would be assumed by a reader or listener that the deaths are those of people, so that The flood which happened last week caused 100 deaths would be a more natural way of saying it, and Last week's flood killed 100 people even more natural. The test sentence is an awkward, non-fluent one which may have been composed by a non-native speaker.
Josef Stalin, a famous former Soviet dictator, is often quoted as saying "The death of one man is a tragedy. The death of millions is a statistic."
There's no proof that Stalin ever said this, but even if he did, he would likely have been quoting a 1932 essay on French humour by a German journalist, satirist, and pacifist Kurt Tucholsky, who quotes a fictional diplomat from the French Ministry of Foreign affairs, speaking on the horrors of war.
"The war?" says Tucholsky's diplomat, "I cannot find it to be so bad! The death of one man: this is a catastrophe. Hundreds of thousands of deaths: that is a statistic!"