What part of speech is walking in the above sentence?

Which of the following best demonstrate a strong conflict? a. catherine enjoyed painting, and was able to practice her art every tuesday. b. leanne loved camping, but it was a lot of work. she had to start gathering her supplies weeks before the big trip. c. kate wanted to be a fitness instructor. she worked hard to become a certified aerobics teacher. d. russell knew that if he did not get a good grade on his test, then he would fail the class and not be allowed to graduate.