Kilogram
Metric ruler
The gram.
The kilogram.
Grams.
Pounds.
I would use grams.
The unit of weight is the newton, since a weight is a force. However, when people say "weight" they often mean "mass" (or confuse weight with mass); and it is common to talk about a person's mass (or incorrectly, a person's "weight") in kilograms.
An appropriate metric unit of mass for a pencil would be grams.
You would use kilograms (kg) to measure someone's mass in metric units.
Weight is a force, so weight is measured in newton. However, when people talk about a person's "weight" they usually mean its "mass" - people tend to confuse that. Mass is measured in kilograms.
In physics, mass is usually measured in kilograms. Metric weighing-scales will usually display mass in kilograms, so this is a likely choice for the measurement of a person's mass.
The Newton. However, "weight" and "mass" are often confused; if you want a person's mass, that would be measured in kilograms.