The norm N(a+b√−17)=a²+17b² is multiplicative. The element has norm 21; a nontrivial factorization would require factor norms 3 and 7, neither represented by a²+17b². It is irreducible. But it divides 21=3×7 and divides neither 3 nor 7, since 21 divides neither norm 9 nor norm 49. It is not prime. **B**.
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