Answer: a. nebula
A nebula is a large cloud that is formed by dust and gas present in space. Some of the nebulae may also form by the explosion of a dying star called as supernova. Also, some nebulae may prove as a precursor of new star formation. The gases which are responsible for the formation of nebulae are helium and hydrogen. The dust and gases which forms the nebula are far but they are brought together as a single mass by the action of gravity. The clumps of gases and dust get bigger and the gravitational force acting on them also increases.