Density requires mass and volume measurements

Steps:

• Density formula is mass divided by volume; mass needs a balance (3).
• For irregular shape, volume measured by water displacement in a measuring cylinder (1).
• Stop-watch (2) measures time, irrelevant for density.
• Thus, only 1 and 3 are needed.

Why C is correct:

• It includes balance for mass and measuring cylinder for volume via displacement method.

Why the others are wrong:

• A includes unnecessary stop-watch.
• B omits balance, can't measure mass.
• D omits measuring cylinder, can't find volume of irregular object.

Final answer: C