Final answer:
Among the options provided, 'People with symptoms of radiation exposure' is the most indicative of a nuclear attack, as radiation can cause serious damage to living cells, leading to illness or death.
Step-by-step explanation:
Nuclear attacks result in the release of high-energy particles and electromagnetic waves. If these encounter living cells, they can break chemical bonds or ionize molecules. The resultant biological damage can be extensive as alpha and beta particles emitted from nuclear reactions cause ions and molecular fragments to become highly reactive. This interrupts normal cellular processes and can lead to serious health consequences, including illness or death.
The immediate signs of radiation exposure include symptoms such as nausea, vomiting, burns, and hair loss. Hence, if a large number of individuals suddenly present with symptoms associated with radiation poisoning, this could indicate exposure resulting from a nuclear attack.