Final answer:
Molybdenum, specifically Molybdenum-99, is the element from the options given that is generated in a nuclear reactor.
Step-by-step explanation:
The student's question pertains to which elements can be generated within a nuclear reactor. Among the options provided, Molybdenum is the element that is reactor-generated.
Specifically, the isotope Molybdenum-99 is produced in a nuclear reactor by the neutron bombardment of Molybdenum-98, which itself is a stable isotope of molybdenum.
In the broader context of nuclear chemistry and medical applications, Molybdenum-99 decays to technetium-99m, an isotope extensively used in nuclear medicine for imaging and diagnostic purposes. Neither Thallium nor Indium are primarily produced in reactors.