The most accurate response is:
The primary consumer took in 5,000 units of energy and released 4,950 units, leaving 50 units for the secondary consumer to take in.
In this scenario, the energy transfer corresponds to the typical energy losses that occur in an ecosystem, primarily through metabolic processes and heat. The producer had 5,000 units of energy, and when consumed by the primary consumer, not all of that energy was retained; a significant amount was released, leaving the secondary consumer with the energy they could use.