Explanation:

- Scientists at Cambridge University are ________ how plants can give us sustainable energy.

- Option 1: looking

- Doesn’t convey detailed analysis or study.

- More passive and less technical.

- Option 2: investigating

- Suggests a thorough and systematic exploration.

- Commonly used in scientific contexts.

- Shows active search for new knowledge.

- Option 3: scrutinizing

- Implies a very critical and detailed examination.

- Might be too intense for initial research stages.

- Option 4: inspecting

- Usually refers to checking or examination for particular purposes.

- Less about detailed scientific study.