Density-dependent inhibition is explained by which of the following? As cells become more numerous, the protein kinases they produce begin to compete with each other, such that the proteins produced by one cell essentially cancel those produced by its neighbor. As cells become more numerous, the cell surface proteins of one cell contact the adjoining cells and they stop dividing. As cells become more numerous, they begin to squeeze against each other, restricting their size and ability to produce control factors. As cells become more numerous, the level of waste products increases, eventually slowing down metabolism.

Regulation of cell division includes various controlling processes that inhibit uncontrolled cell division, to prevent development of tumors. Density-dependent inhibition involves reduction in cell division, when a population of cells acquires certain density. It leads to contact between cell surface proteins of adjoining cells that gives negative signals for cell division and thus, prevent cell division.

Thus, the correct answer is 'as cells become more numerous, the cell surface proteins of one cell contact the adjoining cells and they stop dividing.'