Answer:
Flybys.
Step-by-step explanation:
Of the given options the cheapest type of mission of a robotic spacecraft is
Flybys.
Flybys are gravity assisted( they are accelerated by the force of gravity of Earth or the planet they are meant to orbit ) and hence require less fuel. NASA's Juno Mission to Jupiter in 2011 was a flyby. The spacecraft is studying planets composition, gravity field, magnetic field, etc.