Answer:
Researchers at the Occidental Research Bureau have struggled for three decades to produce an artificial leaf able to absorb CO₂ from the atmosphere and convert it into chemical energy. Yet now it seems that they have at long last achieved their goal.
Explanation:
The correct auxiliary in that sentence is have since it is referring to the researchers. In other words, it is a third person in the plural, so the correct auxiliary is have.
The term man-made is not wrong, but it would be better to use the word artificial, which means that nature did not create it.
