The adjectives - empty, excellent, circular, extreme, chief, entire, complete, perfect, final, last, unique, universal, round, square, triangular, eternal, etc. are not used in the comparative or superlative degree.
So, 'a round stone' (as in option (d)) is the correct usage.
Hence, option (d) is the answer.
Note: · The Positive Degree is used to denote the mere existence of some quality of what we speak about. It is used when no comparison is made.
· The Comparative degree denotes a higher degree of quality than the positive and is used when two things are compared.
· The Superlative degree indicates the highest degree of quality and is used when more than two things (or sets of things) are compared.
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