Final answer:
The stinging plant of the genus Urtica is Urtica dioica (option A), known for its stinging hairs that cause irritation upon contact with skin.
Step-by-step explanation:
The plant of the genus Urtica known for its stinging hairs is Urtica dioica, commonly referred to as the stinging nettle. Urtica dioica's leaves and stems are covered in tiny, hollow hairs filled with a mixture of chemical irritants, including histamine, acetylcholine, and serotonin. When these hairs come into contact with skin, they can cause a stinging sensation and a red, itchy rash. This is a defense mechanism to protect the plant from herbivores.