Option A is correct because both the assertion and reason are true, and the reason correctly explains the assertion. According to the context, embryonic stem cells are pluripotent in nature, meaning they have the potential to differentiate into almost all cell types of the embryo. The context explicitly states that pluripotent stem cells 'have the potential to differentiate into almost all cell types of the embryo except the cells of the extra embryonic support tissues.' Additionally, it confirms that embryonic stem cells 'have the ability to give rise to all the three germ layers, i.e., ectoderm, mesoderm and endoderm' and can 'differentiate into multiple tissue types.' Therefore, being pluripotent is precisely why embryonic stem cells can give rise to different cell types, making the reason the correct explanation of the assertion.