The mistake was that when substituting in the value for d, the student used the value for the radius instead.
For this to be correct, they should double the value of the radius (so do 5.1 × 2) to give them 10.2, and then work out the question:
C = π × d
C = 3.14 × 10.2
C = 32.028 ≈ 32.03 m (rounded to the nearest hundredth)