Final answer:
Well-formed crystals with distinct faces are known as euhedral crystals, which is the correct answer to the question.
Step-by-step explanation:
Crystals that exhibit well-formed faces and precise geometric shapes are referred to as euhedral crystals. Such crystals have flat, distinct, and visible faces that often reflect the internal symmetry of their crystal lattice. When these well-defined faces are missing, the crystal is termed anhedral, which means it lacks the clear facets that characterize euhedral specimens. A crystal that has some form but not all sides well developed is called subhedral. Therefore, the correct answer to the question is option d. euhedral.