The correct option is
D are requested
The given notice is an invitation to the students to attend an inter-school volleyball match.
The subject of the sentence is the word, 'students'. The clause, 'Students _______ to come' is in passive voice because the subject of the sentence i.e. 'students' is not the doer of the action word, 'request'.
In passive voice, the suitable form of the auxiliary verb, 'be' is followed by the past participle form of the main verb i.e 'requested'.
The subject of the sentence i.e., 'students', is in the third person and in plural form. The appropriate form of the auxiliary verb, 'be' which can be used is: 'are'. The suitable form of the verb which can fill the blank no. 9 is: 'are requested'. Hence, option D is correct.
Option A: In this option, the suitable form of the auxiliary verb, 'be' is missing. Only the past participle form of the verb (requested) is given. Hence, this option is incorrect.
Option B: As the subject is in plural form, the auxiliary verb, 'is' cannot be used. Hence, the verb form, 'is requested' is incorrect.
Option C: In this option, the present continuous form of the verb i.e 'requesting' is used. In the passive voice, the past participle form of the main verb i.e 'requested' has to be used. Hence, this option is incorrect.