Answer: A) mourning
Step-by-step explanation:
Mourning is the way of expressing intense sadness and sorrow for a dead person through any specific act such as wearing black clothes, keeping silence and praying etc. These acts represents the the grief and loss a person feels for the died person.
According to the question, Sarah's classmates and teacher are also mourning for Sarah's death through tying black band on arm to express their sorrow.
Other options are incorrect because depression is going into severe sad verge of long duration in an individual .Bargaining is negotiation technique.Thus, the correct option is option(A).