Answer:
C) started into the process during the period.
Step-by-step explanation:
Since materials are added at the beginning of the production process, all units started in the process during the period will be completed at least regarding materials, so the total equivalent units for materials = number of units started in the period.
For example, you are manufacturing doors, all the wood necessary to produce each door is added at the moment the production process starts, so it doesn't matter how advanced the conversion process is, all the materials will be included.