The correct answer is: D. Size of aggregate.
Workability is the ease with which concrete can be mixed, placed, and finished. It is affected by a number of factors, including the water-cement ratio, the amount of air in the mix, and the size of the aggregate.
The size of the aggregate has a significant impact on workability. The larger the aggregate, the less workable the concrete will be. This is because larger aggregates require more water to be mixed with them, which makes the concrete more difficult to place and finish.
The other options are not inversely proportional to workability. Time of transit does not affect workability, as long as the concrete is not allowed to dry out. The water-cement ratio affects workability in a direct way, as a higher water-cement ratio will result in a more workable concrete. The amount of air in the mix affects workability in a direct way, as a higher air content will result in a more workable concrete.