The correct answer is option 'C' - 0.3%.

Whitney's theory of ultimate load method is a method used to determine the ultimate load carrying capacity of reinforced concrete structures. It is based on the assumption that concrete reaches its ultimate strain of 0.3% before failure.

Here is an explanation of why the ultimate strain of concrete is assumed to be 0.3% according to Whitney's theory:

1. Whitney's theory:
Whitney's theory is based on the observation that concrete behaves as a linear-elastic material up to a certain point, after which it starts to exhibit non-linear behavior before failure. The theory assumes that the ultimate strain of concrete is reached just before failure.

2. Strain in concrete:
Strain is a measure of deformation in a material and is defined as the ratio of the change in length to the original length. In the case of concrete, strain is typically measured in terms of the change in length per unit length.

3. Ultimate strain:
The ultimate strain of concrete refers to the maximum strain that concrete can undergo before failure. It is an important parameter in determining the load-carrying capacity of a structure, as it indicates the limit beyond which the concrete will no longer be able to sustain the applied load.

4. Experimental studies:
Experimental studies have shown that the ultimate strain of concrete typically ranges from 0.2% to 0.4%. However, for design purposes, a value of 0.3% is commonly used as a conservative estimate.

5. Safety factor:
In structural design, it is important to consider safety factors to account for uncertainties and ensure that the structure can withstand the applied loads. By assuming a conservative value of 0.3% for the ultimate strain of concrete, Whitney's theory provides an additional safety margin to ensure the structural integrity of the concrete element.

In conclusion, according to Whitney's theory of ultimate load method, the ultimate strain of concrete is assumed to be 0.3%. This assumption is based on experimental studies and provides a conservative estimate to ensure the safety and structural integrity of reinforced concrete structures.