Choice "B" is correct. Generally, agency power may be granted orally, even if the agent enters into contracts that must be in writing to be enforceable. However, most states require an agency agreement to be in writing if the agent is to purchase or convey interests in land.
Choice "a" is incorrect. Generally, agency power may be granted orally. The power to buy office supplies need not be granted in writing.
Choice "c" is incorrect. Generally, agency power may be granted orally. The power to hire persons to perform services need not be granted in writing. (Note that repairing a building involves a service and not an interest in land.)
Choice "d" is incorrect. Generally, agency power may be granted orally, even if the agent is to enter into contracts that must be in writing to be enforceable. The power to hire persons to perform services need not be granted in writing.