biological combustion industrial lightning
I gave my students this multiple choice question: In order for nitrogen to be used for growth in organisms, it must be "fixed." When nitrogen is "fixed," it is combined into the form of ammonium or other nitrogen ions. During which type of fixation do organisms and nitrogen interact?

Only one answer is correct among these 4 choices:
- biological
- combustion
- industrial
- lightning

Could you tell me which one is correct?

biological