Which comma rule does this student need to apply to this passage?

i really thought she loved me but i discovered her grave betrayal one afternoon. it was easy to figure out because she accidentally sent a text to me that was meant for him. i was confused by the text because it said, "don't tell mike." i felt insulted that she thought she could get away with this infidelity. at first i felt very angry but then i felt so hopeless and depressed.

a. use a comma to separate three or more items in series .
b. use a comma and a conjunction to separate two complete thoughts .
c. use a comma to set off a phase or clause at the beginning of a sentence
d. use a comma to separate a coordinate adjective.