Final answer:
The principal square root of 64 is the positive root, which is 8, as per convention when seeking the principal square root.
Step-by-step explanation:
The principal square root of 64 is the positive square root, which is 8. This is because when you multiply 8 by itself (8 x 8), you get 64.
The other square root, -8, is also mathematically correct as (-8) x (-8) equals 64, but when a question asks for the principal square root, it is conventionally seeking the positive one.
Hence, the correct answer is d. 8.