Which tool would be the most precise for measuring the height of plants in millimeters?

The metric ruler is the most precise tool for measuring the height of plants in millimeters because it is specifically designed for linear measurements with clear and accurate markings. Rulers typically have millimeter increments, allowing users to measure small distances accurately. This precision is essential in biological studies, where even slight variations in height can be significant in data collection and analysis.

In contrast, while a measuring tape can measure longer distances, its flexibility may introduce inaccuracies when measuring smaller heights. A caliper, although very precise for small measurements, is more commonly used for measuring the dimensions of objects rather than plant heights. A balanometer is a specialized tool used to measure mass or weight in a specific way and is not applicable for measuring height at all. Thus, for the task of measuring plant height in millimeters, the metric ruler stands out as the most effective and precise option.