Explanation:

- The sentence suggests that solar energy development is significant because it could replace oil, implying widespread importance.

- Option 1: compound - This refers to something consisting of two or more parts; it doesn't fit the sentence context.

- Option 2: usual - This means common or ordinary, which doesn't capture the sense of wide importance.

- Option 3: global - This means worldwide, encompassing or affecting the whole world, fitting perfectly with the context of solar energy as a widespread interest.

- Option 4: simple - This means easily understood or uncomplicated, not reflecting the broad significance implied.