The maximum possible of length of BD is possible when angle BCD is 180 degrees
BDmax = 10
At maximum BD, angle BAC = 60 (because it would form an equilateral triangle)
But in a quadrilateral we can’t have one of the sides as 180 degrees
Thus, BD would reduce in length
If BD is reducing length, then to accomodate that angle BAD must also reduce
The only answer smaller than 60 is 45
Thus, ans. (A)