Transgenic mice carry a foreign gene a. only in their germ line cells. b. only in their somatic cells. c. in some cells (both germ line and somatic) but not in others—transgenic mice are chimeras. d. in all of their cells.

The correct answer is:

d. in all of their cells.

Transgenic mice carry the foreign gene in all of their cells, including both germ line and somatic cells, if the gene is successfully integrated into their genome. This means that the transgene can be passed on to the next generation through their germ cells.