For stairs spanning horizontally, the minimum waist provided is

Answer: Option B

In RCC stair design, the waist is the minimum thickness of the inclined slab measured perpendicular to the soffit of the stair.

For stairs spanning horizontally, the slab behaves like a reinforced concrete slab and must have sufficient thickness to safely resist bending moments and shear forces.

As per standard RCC design practice, a minimum waist thickness of 12 cm is required to ensure adequate strength, stiffness, and durability.

Providing a waist thickness less than this may lead to excessive deflection and structural inadequacy.

Therefore, the correct answer is Option D – 12 cm.