Final answer:
The Challenger space shuttle disaster was due to insufficient testing of the prototype, particularly the O-rings in the solid rocket boosters.
Step-by-step explanation:
When the Challenger space shuttle exploded, the design step that had not been thoroughly executed was perform tests on the prototype. This critical step involves evaluating the prototype rigorously to ensure that all aspects of the design work as intended and can withstand operational stresses. For the Challenger, there was a failure in the solid rocket booster's O-rings, which was identified as a potential issue but not adequately addressed. The importance of prototype testing is to uncover and correct flaws before the final product is deployed, a step which, in the case of Challenger, proved to be devastatingly overlooked as the concerns about the O-ring's performance in cold temperatures were not fully acted upon.