Given: They . . . . (return) as soon as possible.
To determine the correct answer, we need to consider the context and the verb form that is suitable for expressing future actions.
Option A: can return- The verb form "can return" indicates the ability or possibility to return.
- However, it does not convey the certainty or intention of returning in the given context.
- Therefore, option A is not the correct answer.
Option B: may return- The verb form "may return" indicates the possibility or permission to return.
- While it suggests the possibility of returning, it does not express a strong intention or certainty.
- Therefore, option B is not the correct answer.
Option C: will return- The verb form "will return" is used to express a future action with certainty and intention.
- It indicates that the subject has a definite plan or intention to return as soon as possible.
- Therefore, option C is the correct answer.
Option D: none of the above- This option is incorrect as option C, "will return," is the correct answer.
In conclusion, the correct answer is option C: "will return."