Final answer:
Electrons were first discovered in a cathode ray tube by J.J. Thomson, who used it to demonstrate the properties of electrons. Cathode ray tubes enabled the identification of electrons as negatively charged particles.
Step-by-step explanation:
Electrons were first discovered in a cathode ray tube, a type of gas discharge tube used in television sets for decades. When an electric current was passed through this tube, a glowing beam known as the cathode ray traveled from the negatively charged cathode to the positively charged anode. This beam was composed of particles that scientist J.J. Thomson identified as electrons.
Thomson's work began in 1897, and in his experiments with cathode ray tubes, he demonstrated the presence of electrons by showing that they carried momentum and that their path could be bent by a magnetic field in a way that was consistent with negatively charged particles. The use of a cathode ray tube thus played a pivotal role in the discovery of the electron. Therefore, the correct answer to the question is A) Cathode Ray.