With regards to the restrictions on taxing powers of the states, which of the following statements is correct?
(a) A state legislature can impose tax on the supply of goods meant for export. (b) A state legislature is not empowered to impose tax on the sale of electricity which is consumed by the Centre. (c) A state legislature is not allowed to impose taxes on professions, trades, callings and employments. (d) None of the statements given in (a), (b) or (c) above is correct.