Here's a problem to solve: Spirogyra are green algae that can reproduce sexually. Which of the following features identifies reproduction in Spirogyra as sexual reproduction?

Among the 4 following options, which is the correct answer?

- A: The cells of parent algae have nuclei.
 
- B: Each offspring contains chloroplasts.
 
- C: Several offspring may be produced at once.
 
- D: Genetic material is contributed by two parent cells.
 