The Smithsonian's National Air and Space Museum says:
By the early 1930s, airlines were introducing distinctive uniforms for their employees, and women were entering the ranks of flight attendants.
Pilots were given military-style uniforms to reflect their status. Pan American emulated luxurious ocean liner service by calling its flying boats "Clippers" and its pilots "Captains," and attiring its crews in naval-style uniforms with white hats and navy-blue, double-breasted jackets and rank insignia on the sleeve cuffs. Other airlines followed suit. Many of these customs continue today.
This is why choice b is the correct one. A military-derived uniform projects importance / reflects [high] status. While a flight jacket—or a cap that reads 'pilot' for that matter—might adequately serve the purpose of identification (choice a).
Look no further than Wilhelm Voigt, an imposter in an officer's uniform can command real soldiers, place people under arrest, and make away with a treasure.
(Navy Officer, Pilot) Side-by-side comparison.