The correct option is A Subject + simple future of the verb 'to be' + present participle
A is correct as simple future tense is used to express an action that will take place in the future. It is generally reserved for something that will begin and end in the future. It's a statement of fact, made with certainty. As simple future of "to be" is "will be" this is correct.
B is incorrect as "to have" becomes "will have" and is used in future perfect with past participle.
C is incorrect as future of "to do" is "will do" and it is grammatically incorrect with present participle (ing) form.
D is incorrect assimple future tense does not take the past participle form with "will be".