Answer:
exposure of a photographic plate is the correct answer.
Step-by-step explanation:
Henri Becquerel discovered radioactivity by his studies of uranium and he was particularly interested in uranium and its substance.
He unexpectedly noticed that When uranium-rich element named pitchblende releases invisible and penetrating radiations that darken the photographic plate.
To find whether the phosphorescent uranium salts may absorb the sunlight and emit it again in the form of X-ray or not he used photographic plates.
Thus Exposure of a photographic plate helped Becquerel determine uranium radiates rays.