In which sentence is whom used correctly?

a) whom took those pictures?

b) by whom were those pictures taken?

c) do you think he is whom took those pictures?

d) whom has the skills to take these pictures?

Letter A is wrong.

Who substitutes pronouns like he, she or it and whom substitutes him, her and others alike. "He took those pictures?" would have been fine, and so would "Who took those pictures?" Same goes for letter C and D. If you can fit He or She, you should use Who.

B) By whom were those pictures taken?

This is the correct answer. You can see that by rearranging the sentence.

"Those pictures were taken by whom?"

in "letter from a birmingham jail," king argues that there are four basic steps in any nonviolent campaign: "[c]ollection of the facts to determine whether injustices exist; negotiation; self purification; and direct action." finally, direct action is taken and acts such as boycotts and sit-ins are staged.