ANS:D - all the above.

1. Clear spacing between ribs shall not be greater than 4.5 cm: This condition is essential for ensuring proper load distribution and structural integrity. Smaller spacing between ribs helps in distributing loads more effectively and reducing the risk of excessive deflection or cracking.
2. Width of the rib shall not be less than 7.5 cm: A minimum width for the ribs is crucial to provide adequate stiffness and load-carrying capacity. Wider ribs can better support the slab and transfer loads to the supporting beams or columns, enhancing structural performance.
3. Overall depth of the slab shall not exceed four times the breadth of the rib: This condition limits the overall depth of the ribbed slab relative to the width of the rib. It helps maintain structural efficiency and prevents excessive bending moments and deflections.