Is this a typo (in a Practice Question)?

Because today’s students can expect to hold multiple jobs—some of which may not even exist yet—during [Q44] our lifetime, studying philosophy allows them to be flexible and adaptable. High demand, advanced exam scores, and varied professional skills all argue for maintaining and enhancing philosophy courses and majors within academic institutions.

I got this from an official SAT book, practice exam. Question is: What to replace OUR with? Answers: No Change, One’s, His/Her, and Their.

The answer is Their, which I understand, except for one part: shouldn’t it be “Their LifetimeS” instead of Their Lifetime as is placed in the passage?

The choice of singular or plural for “lifetime” depends on the context. If the passage is talking about the students’ lifetimes as multiple types of lives that vary across the students, then it should be plural.
But here the life experiences of the students are described in a way that refers similarly to all of the students - they will be expected to hold multiple jobs - and so the “lifetime” is singular. During the students’ lifetime, they are expected to experience multiple job changes. Tricky.

Regardless of whether there’s a mistake, the answer can only be “their” because the sentence begins talking about “today’s students” (plural). Also, “One’s” and “His/Her” are likely interchangable answers so get eliminated. That already reduces the choice to two possibilities, and it can’t be “Our” (no change) since the sentence wasn’t referring to students in plural first person.

A long answer to your question.

Good question (and replies)! In terms of a reply in a different form, would you change this? “During the dinosaurs’ lifetime on Earth . . .”

Sometimes abstract nouns like “lifetime” can be singular and seemingly violate the usual noun-number agreement rules. They will never test you on those, of course, but they may appear in the flow of a passage (just like other rules are occasionally broken in passages).