About 4.5 billion years ago, what major event occurred in the solar system?

• A. The solar system formed
• B. The Earth reached its current size
• C. The Moon acquired its oceans
• D. Oxygenic photosynthesis began

Answer: (A)

The event being tested is the birth of the solar system itself. About 4.5 billion years ago, a collapsing cloud of gas and dust gave rise to the Sun and a surrounding protoplanetary disk from which planets, moons, and other bodies formed. This timing is supported by radiometric dating of the oldest solar system materials, such as certain meteorites, which cluster around 4.56 billion years. So the major event at that time is the formation of the solar system.

The other statements don’t fit as a single, defining moment at that time: Earth reaching its current size happened through ongoing accretion over tens of millions of years after the Sun formed, not at a single instant. The Moon has no oceans in the sense of Earth-like bodies of liquid water, so that wouldn’t be the event described. Oxygenic photosynthesis began much later, around ~2.4 billion years ago, leading to changes in Earth's atmosphere long after the solar system’s birth.