You are given a math word problem and you are supposed to make use of mathematical concepts like probability, combinatorics, linear equations, sequences/series, or simple operators like addition, subtraction, division, or multiplication to answer the given question. Choose the best option from the 5 given options and report that option.

Question: Consider a quarter of a circle of radius 81. Let r be the radius of the circle inscribed in this quarter of a circle. Find r.

Option A: 86*(sqr2 -1

Option B: 88*(sqr3 -1

Option C: 4*(sqr7 - 1

Option D: 81* (sqr2 -1

Option E: None of these
Option D