Final answer:
Discovery (a), involving a gas giant very close to its parent star with no terrestrial planets, would most challenge the nebular hypothesis since it contradicts the conventional view of giant planet formation requiring stable water ice far from the heat of a star.
Step-by-step explanation:
The discovery that would most strongly challenge the nebular hypothesis for the formation of solar systems is (a) A solar system with a gas giant very close to its parent star and no terrestrial planets. The nebular hypothesis suggests that giant planets form at a considerable distance from their stars where it is cold enough for ice to condense. Therefore, the presence of hot Jupiters, or gas giants that orbit close to their stars, requires a revision to this hypothesis, suggesting the possibility of planet migrations or gravitational interactions altering their orbits after formation.
The nebular hypothesis is built upon the idea of planet formation through the accumulation of material in a circumstellar disk and typically entails the formation of ice giants at a distance where water ice can form. Thus, the existence of hot Jupiters contradicts the traditional view of where and how these massive planets could form according to the initial hypothesis. However, even with such interesting systems, the notion of planet migration within protoplanetary disks helps to reconcile observed solar systems with the nebular hypothesis, accommodating modern discoveries such as gas giants in close orbits to their stars and those with opposite or perpendicular orbits.