The Correct Answer for by the time cynthia books the flight to iceland, she _______ her sister about the visit.
Answer: D is the correct option. Therefore ''will have called'' must be taken.
This is true because it expresses a prior action before the time at which you will book the flight. This means that Cynthia will have to call her sister before booking the flight.
Apparently there are two things Cynthia needs to do. One is communication and the other is flight booking, but all of them occur in the future. Therefore, to properly express this contextual meaning in the sentence, "shall be called" should be used.
In the first option, we see the use of the irregular singular verb, "a" with the past participle "called" in combination as "called". According to the sentence, the use of such a plural of the verb does not express the future action that Cynthia will take.
Indeed, "can be called" places each verb across the subject of the sentence, cynthia, in the past tense while the two actions to be performed are for the future, thus requiring a future and not a past tense.
Again, the "will called" in the second option has only one problem, the shedding of the irregular singular verb, "a," which falls between the will and called. However, this is a grammatical error. The irregular verb must be plural "have" because it comes after the auxiliary verb "will".
Therefore, "will be called" is the correct answer. It is grammatically well structured and expresses the future action that the subject, Cynthia, must take.
The correct sentence would now be "By the time Cynthia books the flight, she will have called her sister about her visit."