What observes the arising and passing away of consciousness?
Consciousness is dependent upon the six Sense Bases.
It is the process of Perception that understands the arising and passing away of consciousness.
This perception is dependent upon Feeling.
Feeling is dependent upon Contact.
Contact is dependent upon the six Sense Bases.
The observation that arises happens through Mentality, which encompasses the faculties of Contact, Feeling, Perception, Intention and Attention.
It is specifically Attention through which the arising and passing away of Consciousness is observed.