A radioactive marker was applied to the mesoderm of a mammalian embryo. which tissues or organs would have this marker in the adult?

Bones and muscles would have this marker in the adult. Mesoderm is responsible for the development of the middle layer of skin, which leads to the formation of connective tissues and muscles of the body. This includes blood, cardiac, skeletal, smooth muscles etc.

The mesoderm is one of the germ layers present in an embryo and it develops primarily into muscles and organs. The radioactive marker would therefore be visible in the adult organism's cardiac muscle cells, smooth muscle cells, skeletal muscle cells, red blood cells and many other such specialized cells.