Choice "C" is correct. An unrelated business does not include any activity where all the work is performed by the organization by unpaid volunteers.
Choice "d" is incorrect. Receipt of unrelated business income in excess of certain limits does not affect the organization's exempt status, but does trigger a tax on income from excess holdings.
Choice "b" is incorrect. Unrelated business income relates to both the performance of services and the sale of goods that is not substantially related to the exercise or performance of the entity's purpose or function.
Choice "a" is incorrect. Unrelated business income tax is assessed on the taxable income from any unrelated business. Unrelated business taxable income is the gross income derived from the unrelated trade or business less directly connected allowable deductions.