Essential apparatus for measuring magnesium-HCl reaction rate

Steps:

• Determine fixed mass of magnesium requires a balance for accurate weighing.
• Measure fixed volume of hydrochloric acid needs a measuring cylinder for precise volume.
• Track reaction rate involves timing the reaction duration or gas evolution, requiring a stop-watch.
• Temperature control is not specified, so thermometer is unnecessary.

Why A is correct:

• Balance measures magnesium mass, measuring cylinder volumes acid, and stop-watch times rate, directly enabling fixed quantities and rate determination per experimental design.

Why the others are wrong:

• B lacks measuring cylinder, preventing accurate acid volume measurement.
• C omits balance, making magnesium mass unmeasurable.
• D includes unnecessary thermometer, as temperature variation is not required.

Final answer: A