Final answer:
The Vastus lateralis muscle is the best site for an intramuscular injection in a one-year-old child due to its size and distance from major nerves and blood vessels.
Step-by-step explanation:
The best site for giving a one-year-old child an intramuscular injection is option D) the Vastus lateralis muscle. This muscle is the preferred site for intramuscular injections in infants because of its large size in proportion to the rest of the child’s body and because it is well away from major nerves and blood vessels. The deltoid muscle is often too small in infants for an intramuscular injection and is more suitable for older children and adults. The ventrogluteal and dorsogluteal sites are not recommended for infants due to the risk of damaging nerves or blood vessels in the area.