The only way to captivate the audience is to surprise them constantly with new scenes and new actors before they completely lose interest in the play.
1st split in here is "the audience" which need to be defined either plural or singular noun. Regarding to parallelism, it is insignificant in this case since all the answer choice correctly follows the rules.
2nd split lays on the intended meaning of the sentence.
A. to surprise them constantly with new scenes and new actors before they completely lose interest in the play.--->This is the correct answer, "the audience" should be in plural form, and this option conveys the intended meaning.
B. if they are surprised constantly with new scenes and new actors before they completely lose interest in the play.--->"the audience" should be in plural form; however, it doesn't convey the intended meaning.
C. for them to be surprised constantly with new scenes and new actors before they completely lose interest in the play.----->"the audience" should be in plural form, wordy.
D. if the audience is surprised constantly with new scenes and new actors before they completely lose interest in the play.---->"the audience" should be in plural form, wordy and not convey the intended meaning.
E. to surprise it constantly with new scenes and new actors before it completely loses interest in the play.---->"the audience" should be in plural form.