A human may go from being microscopic to being six feet tall in less than 16 years. What change on the cellular level accounts for most of the change in size?

the number of cells increases

Explanation:

The changes experienced by a human, from birth to 16 years old in form of growth is facilitated by the increase in the number of cells.

Most tissues in the human body grow by increasing their cell number, but this growth is being monitored and regulated to maintain a balance between the different tissues.

since there are no options posted i'll give you a few examples.

a dolphin's flipper, bird's wing, cat's leg.