Final answer:
The question about the optional calculation method for electric space-heating units cannot be accurately answered with the options provided, as the NEC specifies using 100% for the first 10 kW and 75% of the remainder for four or more units.
Step-by-step explanation:
According to the optional calculation method described in the National Electrical Code (NEC), when calculating the load for four or more separately-controlled electric space-heating units, we typically use 100% of their nameplate ratings for the first 10 kW and 75% of the remainder. However, the specific percentage is not stated in the question options (a. 80% b. 100% c. 120% d. 150%). Therefore, without additional context or reference to a specific article of the NEC, the question cannot be answered accurately as posed just using the provided options.