ANS:B - 10%

As I know, A good building stone should not absorb more than 15% of water. But, there is mentioned that a good stone should not absorb water more than 10%.

That means according to your site a stone having 11% absorption of water is not a good building stone. For good building stone I mean stone used in the building should not absorb water than 5% and for just stone, it's 10%. A good building stone should be less porous. It should not absorb more than 5% of water. Any stone absorbing 10% of water should be rejected. Good Stone Water Absorption - Less than 5%.

Stone Rejected if Water Absorption - More than 10%.