Final answer:
After passing through an expansion fan, the total (stagnation) pressure remains unchanged in an ideal, isentropic process. Therefore, the total pressure after the expansion fan is 1 atm.
Step-by-step explanation:
The question asks about the total (stagnation) pressure after an expansion fan, with the static pressure before the expansion wave being 1 atm. In an isentropic expansion, which is the typical assumption in such physics and engineering problems, total pressure is conserved. Therefore, after passing through an expansion fan, the stagnation (total) pressure remains the same as before the expansion, if the process is considered isentropic and there are no losses due to friction or shock waves.
The correct answer is (a) 1 atm, as this is the initial total pressure given before the expansion wave, and in ideal conditions of an isentropic process, it does not change.