Is this sentence fused, comma-spliced, or correct? psychologists study human behavior, researchers have developed many theories on human nature.

The sentence is comma-spliced since the comma was used to combine two independent phrases. The phrase "Psychologists study human behavior," could have been a sentence on it's own. "Researchers have developed many theories on human nature," could have also been it's own sentence. But the comma was used to join them together as one, making it comma-spliced.

D. work on the hardest parts of the paper first and take multiple breaks until he’s finished