Final answer:
For administering vitamin K to a neonate, the vastus lateralis muscle of either the left or right thigh is the recommended injection site.
Step-by-step explanation:
The best site for administering an intramuscular (IM) injection of vitamin K to a neonate is the vastus lateralis muscle of the thigh. This muscle is large and well-developed at birth, making it the preferred site for IM injections in neonates. Therefore, the correct answer is a) Left thigh or b) Right thigh. Injection into the buttocks is not recommended for neonates due to the risk of injury.