Final answer:
The oldest eon in the geologic time scale is the Hadean eon, which predates even the formation of the oldest known rocks on Earth.
Step-by-step explanation:
The oldest eon in the geologic time scale is the Hadean eon. The geologic time scale is divided into four eons with the Hadean being the earliest, followed by the Archean, Proterozoic, and the Phanerozoic, which is the current eon. The Hadean eon marks the very beginning of Earth's history and precedes the formation of the oldest rocks known on Earth.
To answer the question: The Hadean is the oldest eon among the choices provided, which are 1) Phanerozoic 2) Hadean 3) Proterozoic 4) Archean. The Precambrian is a 'super eon' that includes the Hadean, Archean, and Proterozoic eons. When discussing geologic time, it's essential to understand that the scale is based on important tectonic, biological, and climatic events, and the ages are determined through radiometric dating techniques.