Final answer:
The term that refers to the dilation of a vessel using the root 'angi/o' is 'Angiectasis', which combines the root with the suffix '-ectasis', indicating expansion or dilation (option c).
Step-by-step explanation:
Using the root angi/o, which refers to blood vessels, the term that refers to the dilatation (-ectasis) of a vessel is c) Angiectasis. This term combines the root 'angi/o' (vessel) with the suffix '-ectasis' (dilatation), literally translating to the expansion or dilation of a blood vessel. Angiogenesis refers to the formation of new blood vessels, angiosperm classifies a group of plants, and angioplasty is a medical procedure that involves mechanically widening a blocked or narrowed vessel, commonly through the use of a balloon catheter.
Hence, th answer is option c.