One candela is defined as $1/683$ watt of luminous intensity emitted by a source vibrating at—

• a$9{,}192{,}631{,}770$ Hz into 1 steradian
• b$540 \times 10^{12}$ Hz into 1 steradian
• c$299{,}792{,}458$ Hz into 1 steradian
• d$683 \times 10^{12}$ Hz into 1 steradian