B

Step 1: Understand what cathode rays are. Cathode rays are streams of electrons observed in vacuum tubes. They are produced by the process of electron emission from the cathode when it is heated or subjected to a high voltage.

Step 2: Analyze the pressure conditions required for the production of cathode rays. Cathode rays are typically produced in a vacuum, where the pressure is significantly lower than atmospheric pressure.

Step 3: Evaluate the given options in terms of pressure:

• Option A: 2 cm of Hg = 20 mm of Hg (not low enough for efficient electron flow)
• Option B: 0.1 cm of Hg = 1 mm of Hg (suitable for cathode ray production)
• Option C: 0.01 mm of Hg = 0.001 cm of Hg (too low for practical experiments)
• Option D: 1 µm of Hg = 0.0001 cm of Hg (extremely low, not practical)

Step 4: The typical pressure required for producing cathode rays is around 0.1 cm of Hg (1 mm of Hg).

Therefore, the correct answer is Option B (0.1 cm of Hg).