Final answer:
The discovery of 'hot Jupiters' has led to questioning the nebular theory as it implies that significant planet migrations can occur, which was not originally accounted for in the theory's framework. The correct answer is option: b) discovery of a large number of 'hot jupiters'.
Step-by-step explanation:
Among the given options, the discovery of a large number of 'hot Jupiters' was a cause for the questioning of the nebular theory. This is because the existence of these massive planets orbiting very close to their stars challenges the traditional view of planet formation, which held that giant planets could only form far from their stars where water ice is stable. The unexpected position of these hot Jupiters suggests that planets can migrate significantly from their original formation zones.
Astronomers have adapted the nebular theory to account for such migrations after the discovery of exoplanets, recognizing that gravitational interactions could cause significant shifts in a planet's orbit. Therefore, the correct answer to the student's question is 'b) discovery of a large number of 'hot Jupiters'.'