Weight-bearing activities, like weight lifting and walking, can bones become thicker and stronger. which level of structural organization is responsible for creating new bone?
a) tissues
b) organs
c) cells
d) organ systems

C. Cells

Cells when grouped form tissues, when group of tissues are formed together they make organs, some organisms skip from organs to organs systems to form an existing organism as a whole. Describing how is the cells group is the process of multiplication of cell or the so-called cell division, mitosis.This mechanism of each cell produces another cell that binds together and produces again, repeatedly, to be a larger system called now the tissue as explained earlier. How? Through cell division called, mitosis. Excerptmeiosis –cell division specifically in the sex cells. 

Imma gonna say go with the very first answer (: