What is the reason that the phrase "sōle ortō" is correct?

The phrase "sōle ortō" is correct because it utilizes the verb "ōrior," which is a deponent verb. Deponent verbs are unique in Latin; they are passive in form but have an active meaning. "Ōrior" means "to rise," and in the phrase, "sōle" is the ablative of "sol," meaning "sun." The perfect participle form "ortō" is derived from "ōrior," indicating that the action has been completed. Together, "sōle ortō" translates to "with the sun having risen," effectively conveying the idea of the sun having already risen, which captures the passive form of the verb with an active implication.

The other options do not apply in this context. For instance, while the phrase does not refer to a future event (as indicated in one of the choices), the use of a perfect participle illustrates a completed action rather than something that is yet to occur. Additionally, the phrase does not involve an irregular verb form or an active verb, as "ōrior" retains its deponent nature throughout its conjugations.