Ah, but the Buddha says it's not. It seems like the most likely candidate in the list of five, but the Buddha thinks because the consciousness is fleeting, it couldn’t be constant. Contemporary phenomenology also argues likewise. Husserl’s theory famously claims that consciousness is always consciousness about or for something. We call this “intentionality.” Consciousness doesn’t exist for itself, but for something else. Even if we’re reflecting on consciousness, we’re converting consciousness into the object of reflecting.
So, in short, consciousness doesn’t exist independently of its object. :)