Final answer:
An object speeds up in a gravitational field because it is converting gravitational potential energy into kinetic energy, resulting in it gaining speed as it falls.
Step-by-step explanation:
An object observed to speed up in a gravitational field means that it is falling and converting gravitational potential energy into kinetic energy. The correct explanation for this phenomenon is: b) The object is falling in the gravitational field, so it has gained kinetic energy. This process is consistent with the Law of Conservation of Mechanical Energy, which in the absence of friction, maintains that the total mechanical energy remains constant as energy transitions from one form to another.
When an object descends without friction, its gravitational potential energy (APEg) is transformed into kinetic energy (AKE), represented as AKE = -APEg. This transformation results in the object accelerating and thereby increasing its speed. This concept parallels the behavior of a charge in a constant electric field, where its electric potential energy is converted to kinetic energy.