Final answer:
The simplest and most common measuring tool is the tape measure due to its flexibility and capability to measure in both customary and metric units(option a).
Step-by-step explanation:
When choosing measuring tools, the simplest and most common one is typically the tape measure(option a).
A tape measure is quite versatile, capable of measuring in both customary and metric units, making it suitable for a variety of tasks. Its flexibility allows for measuring irregular objects or spanning longer distances that would be impractical with a rigid tool. Tape measures are commonly marked with inches and feet, and in many cases, metric units as well, proving their adaptability in different measuring contexts.