For the purpose of service demand, a sign circuit shall be computed at:

For the purpose of service demand, a sign circuit is computed at 1200 volt-amperes (va). This value is established based on the specific requirements set forth in the National Electrical Code (NEC) and provides a standardized way to calculate the expected load for various types of electrical circuits.

In this context, the 1200 va figure helps ensure that electrical systems can safely handle the demand of lighting and signage without the risk of overload. By using this standard value, electricians can design circuits that comply with safety regulations and prevent potential electrical hazards.

The other options, while they may represent commonly referenced values in other scenarios, do not align with the established calculation for sign circuits per the NEC, leading to their exclusion as the correct answer in this specific context.