Hydraulic lime is obtained by

Hydraulic lime is a type of lime that can set and harden when mixed with water. It is obtained through a specific process. Let's examine each option:

Option A: Burning of lime stone
This option refers to the production of quicklime (calcium oxide) by heating limestone. Quicklime is not hydraulic lime; it is a different product.
Option B: Burning of kankar
Kankar is a type of nodular limestone, and burning it can result in hydraulic lime. This option is correct.
Option C: Adding water to quick lime
Adding water to quicklime results in the formation of slaked lime (calcium hydroxide) and is not the process for obtaining hydraulic lime.
Option D: Calcination of pure clay
Calcination of pure clay results in the production of other materials, such as clay bricks or ceramics, and is not the process for obtaining hydraulic lime.

So, the correct answer is Option B: Burning of kankar. Hydraulic lime is obtained by burning kankar, a type of nodular limestone that has hydraulic properties.