Final answer:
The untrue statement about matter-antimatter engines is that we have no evidence that antimatter exists, when in fact antimatter has been observed in laboratory conditions. The key challenges are producing enough antimatter and storing it safely.
Step-by-step explanation:
The statement about matter-antimatter engines that is not true is B) Matter-antimatter engines would be great in theory, but to date we have no evidence that antimatter even exists. This statement is incorrect because antimatter does exist and has been created and observed in laboratory conditions. Antimatter particles, like antiprotons and positrons, are real and have been detected through various experiments. The actual challenges of creating a matter-antimatter engine involve producing sufficient quantities of antimatter (statement C) and devising a method for storing it without it annihilating with regular matter, as antimatter and matter annihilate upon contact, releasing energy (statement D).
Regarding the energy potential of matter-antimatter reactions, they do represent the most efficient reactions possible in terms of energy release, as indicated by Einstein's equation E = mc² (statement A). Lastly, statement E speculates that spacecraft powered by matter-antimatter engines could potentially reach speeds of more than half the speed of light, which is a theoretical possibility not yet achieved but considered plausible by physicists.