The most appropriate metric unit for the length of a pencil would be centimeters (cm).

Here's why:

* Millimeters (mm) are too small. A pencil is likely to be several centimeters long, making millimeters cumbersome to use.

* Meters (m) are too large. A pencil is much shorter than a meter.

* Kilometers (km) are way too large, used for distances like road lengths.

Centimeters provide a practical and easy-to-understand measurement for the length of a typical pencil.