Solid chemical fuel. Hence, option C is correct.
What is solid fuel?
Solid fuels include peat, wood, and coal and can encompass solid rocket fuels as well as metals. The earliest fuels used by man were nonfossil fuels of wood and oil from plants and fats from animals.
A solid fuel mixes with an adequate amount of liquid fuel to be able to sustain an open burning flame.
If an engineer's greatest concern is the easy maintenance of a rocket throughout the years, the rocket should be designed to use solid chemical fuel.
Hence, option C is correct.
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