In the design of water tank for thickness upto 100 mm, minimum percentage of reinforcement should be -

Correct option is C

For water tanks and other liquid-retaining structures, the minimum percentage of reinforcement is governed by IS 3370 (Part 2): 2009 (Code of Practice for Concrete Structures for Storage of Liquids). According to the code:
For a wall thickness up to 100 mm, the minimum percentage of reinforcement in the concrete section is specified as 0.3% of the gross cross-sectional area.
This minimum reinforcement ensures:
Crack control due to temperature changes and shrinkage.
Adequate strength and durability for liquid-retaining structures.