Which of the following is not a way in which an author would develop a character?
a. have other characters discuss him of her.
b. write dialouge with the character and another character.
c. let the reader know what the character is thinking and feeling.
d. introduce new events involving the character.

A. have other characters discuss him of her.

Explanation:

Having characters speaking about the main one does move forward the protagonist's arc, since it does not add a relevant layer to his/her actions, reactions, thus, avoiding any possibility of development.

It must be done through him/herself in a variety of ways:

i decided to stay late in the laboratory to continue my experiment with the lightbulb. at midnight, thomas edison opened the door and saw all the stuff he was working on. the lightbulb, film and the transmitter. as he walks in an idea hits him on how the lightbulb would work. he would have to change a few wires around to get it to light up. when he did change the wires around it worked so he decided to write a telegraph to his friend to tell him about his success.   as he was packing up he grabbed the telegraph and walked to his home. he told his wife about the success and the next morning mailed the telegraph.