Stem cells are undifferentiated cells that have the ability to continue to grow and divide in their non-specialized state for long periods of time. although the genes found within the stem cells are the same, stem cells differentiate into unique cell types (e. g., muscle cells, nerve cells, skin cells, etc.) due to a. random segregation and proliferation. b. meiotic division and fertilization. c. mutations in their genetic code. d. different patterns in their gene expression.

The correct answer will be option-D

Explanation:

Stem cells are the cells which have the ability to form different types of cells. They differentiate and specializes to perform different functions in the body and thus forms the different cells.  

The differentiation and specialization are controlled at the gene level by different patterns of gene expression. The pattern of gene expression is not fully understood during development but research have shown that the gene expression is influenced by the different environmental signals and internal regulators.

Thus, option-D is the correct answer.