Question

Which length is usually measured in micrometers?

Answer 1

B length of a cell nucleus

The length of a cell nucleus is very small and would be measured in micrometers. The others, even the earthworm, are significantly larger and would be measured in centimeters, meters, or kilometers.

Answer 2

Any length or distance can be described in any unit of length.
The choice we make usually depends on which unit produces
the most convenient number ... the number that's easiest to
write down, remember, and tell other people about.

The choices that most reasonable people would make are ...

-- millimeters or centimeters for the length of an earthworm,

-- micrometers for the length of a typical cell nucleus,

-- centimeters for the length of the nucleus of unusual cells,
like chicken eggs or ostrich eggs,

-- meters for the length of a football field, 

-- kilometers for the distance across the United States,

-- light-years for the distance between nearby stars, and

-- parsecs for the distance between galaxies or widely-separated stars.