The best unit measure to express the weight of a pen is gram.
Work Step by Step
We would like to use a unit which gives us a comfortable number to work with, that is not too big such as 1,000,000 and that is not too small such as 0.00001. Clearly the units of kilogram and tonne would be too large for expressing the weight of a pen because the number of kilograms or tonnes would be very small numbers. A pen is not so light that we would use units of milligrams since the number of milligrams would be quite a big number. The best unit measure to express the weight of a pen is gram, since most pens are probably several grams in weight. Using the unit measure of gram, we would be able to express the weight of a pen with a number that is comfortable to use.