Based on the information provided in your question about plant hormones and their effects on growth in stems and roots at varying concentrations, the correct conclusion is:
They stimulate maximum stem growth at low concentrations.
This conclusion is supported by the notion that at low hormone concentrations, there is a stimulatory effect on stem growth (suggesting the possibility of a positive correlation), while higher concentrations may lead to inhibition of growth in roots. The statement also does not support equal stimulation in roots and stems at the same concentration.