Torricelli's barometer setup Steps:

• Fill a long, closed tube completely with mercury and seal it.
• Invert the tube into an open dish of mercury, allowing some mercury to flow out.
• Mercury column stabilizes at a height determined by atmospheric pressure balancing the vacuum above.
• Measure the height (typically 760 mm) to find atmospheric pressure in mmHg.

Why C is correct:

• C states the space above the mercury is a Torricellian vacuum, as no air enters and mercury falls due to atmospheric pressure, per Boyle's law and vacuum definition.

Why the others are wrong:

• A: Incorrect, as the tube is initially full, not open to air.
• B: Wrong, mercury doesn't fill the entire tube; it drops to 76 cm height.
• D: False, the height varies with atmospheric pressure, not fixed at full length.

Final answer: C