Final answer:
Neutrinos, subatomic particles created during nuclear fusion in the Sun’s core, serve as direct evidence of fusion because they can be detected on Earth shortly after their production.
Step-by-step explanation:
One direct piece of evidence for fusion in the Sun is the detection of neutrinos. Neutrinos are subatomic particles produced during nuclear fusion reactions in the Sun's core. Unlike photons that take thousands of years to emerge from the layers of the Sun, neutrinos barely interact with matter, allowing them to traverse the Sun's dense interior and arrive at Earth in about two seconds after their creation. This characteristic transparency of the Sun to neutrinos and the ability to detect them on Earth confirms the ongoing fusion processes at the core of the Sun. Studies, such as those involving the Borexino experiment, have specifically identified neutrinos from the Sun's core, further validating solar models of fusion.