The answer should be B, They can all be easily observed.
As all the information says, it is very clear that all his observations can be observed easily, without complicated processes or experiments. Such as the flower color or the seed shape or basically all of the details, all these can be observed directly with eyes in a short time.
On the other hand, we would never know whether the traits are dominant or recessive unless we get more information about the plants' parents or offsprings and/or their genes. Just the observable traits of an individual cannot determine whether their genes controlling for the specific trait is dominant or recessive.
And last but not least, the traits he observed may not be able to be applied to other plants. For example, not all plants are flowering plants and they do not have flowers thus flower color/position cannot be observed.
Therefore, the answer should be B.
