The graph shows the relationship between an athlete's running time and the contribution of three energy systems - HSC - SSCE Personal Development and Physical Education - Question 17 - 2019 - Paper 1

To determine the by-product and rate of recovery for the dominant energy system at 50 seconds, we need to assess the energy systems graph, focusing on the 50-second mark.

At this duration, the predominant energy system is likely the anaerobic lactic system, which primarily produces lactic acid as a by-product.

Recovery from the lactic acid system typically takes longer than recovery from other systems, often ranging between 30 minutes to 2 hours. Thus, the correct option is:

D. Lactic acid: 30 minutes – 2 hours.