The correct option is (d) 11%
For explanation I would say: Stainless steel is a family of iron-based alloys that contain a minimum of 11% chromium. Chromium imparts corrosive resistance to steel. There are 4 types of stainless steel.
They are i. Austenitic stainless steels
ii. Martensitic stainless steels
iii. Ferritic stainless steels
iv. Duplex stainless steels