AP BIOLOGY EXPLANATION PLEASE! <3

Hey guys, I know the correct answer, but I need some assistance understanding WHY it is correct and how to come to that answer! Thank you!

A porcupine eats 3000 J of plant material. Of this, 2100 J is indigestible and is eliminated as feces, 800 J are used in cellular respiration, and 100 J are used for growth and reproduction. What is the approximate production efficiency of this animal?

0.03%

2.7%

3%

33%