Final answer:
Not all mammals have live births; monotremes, including the platypus and echidnas, lay eggs, which is the statement that is not true.
Step-by-step explanation:
The statement about mammals that is not true is: d) They all have live births instead of laying eggs.
Not all mammals give birth to live young; the monotremes, which include species like the platypus and the echidnas, lay eggs.
The other two groups of mammals, marsupials and eutherians (or placental mammals), do give birth to live young.
Marsupials have underdeveloped embryos that continue to grow in the mother's pouch, while eutherians have a more developed form of live birth with the young nourished in the uterus via a placenta.