Final answer:
Goshenite is the best beryl gemstone to simulate a diamond due to its colorless appearance and clarity, which are visually closest to that of a natural, untreated diamond.
Step-by-step explanation:
If you are forced to use a beryl gemstone as an imitation diamond, the best option would be c) Goshenite. Goshenite is the purest form of beryl and is colorless, making it the most similar in appearance to a diamond. Other varieties of beryl like a) Aquamarine, b) Heliodor, and d) Morganite have distinctive colors that would not resemble a diamond as closely. Although beryl and diamonds have different refractive indexes, goshenite's lack of color and greater clarity would make it the closest visual match for simulating an untreated, natural diamond.