Jenna, a first-year teacher, is frazzled at the beginning of class, because she has to take roll, mark the absentee sheet, deal with students who were absent yesterday, and prepare for her class. this best illustrates which aspect of classroom complexity?

That would be simultaneous complexity.
The classroom is a complex environment and teachers usually have to juggle several thing simultaneously, and that is simultaneous complexity. There are also few other complexities such as unpredictability as its not possible to predict all the scenarios that could happen during class so we unpredictable complexity, also because teachers usually work with 25-30 students they have to deal with multidimensional complexity.