Final answer:
Barium is the metallic element with medical and fireworks applications; its compounds, particularly barium sulfate, are used for X-ray imaging of the digestive system and provide green colors in fireworks. Therefore, correct option is a.
Step-by-step explanation:
The reactive, metallic element whose compounds are used in a medical cocktail to outline the stomach and intestines for X-ray examination, and gives green colors to fireworks, is barium (Ba).
Barium belongs to the group of alkaline earth metals which includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium itself (Ba), and radium (Ra). These metals form basic oxides and are somewhat reactive, with divalent metals.
Specifically, barium compounds such as barium sulfate (BaSO4), which are not absorbed into tissues due to their high insolubility, are used medically for X-ray imaging.
This substance, commonly known as a 'barium milkshake' for oral use or a 'barium enema' for rectal use, is a suspension of fine BaSO4 particles in water that provides a clear contrast on X-ray images of the digestive tract. Moreover, barium salts are incorporated into firework formulations to produce a distinctive green color upon ignition.