In interplanetary mission planning, what is a Mars transfer window?

• A. A period when planetary alignments maximize the available sunlight for the journey.
• B. A period when the spacecraft can launch without performing any propulsion maneuvers.
• C. A period when gravitational assists are not involved in the trajectory.
• D. A period when planetary alignments minimize delta-v for a cruise from Earth to Mars via a transfer trajectory.

Answer: (D)