Final answer:
In portrait painting, it is not necessary to match the size of the ear to the eye, nor are the eyes located at the center of the head. Contrast in value is used to create a realistic image against the background. Proper facial proportions maintain a logical relation among all facial features.
Step-by-step explanation:
When painting a portrait with correct proportions, it is important to address several key proportional assumptions. These include the position of the ears, eyes, nose, and mouth. Proportion is the relationship of the sizes of different parts of a face to each other, and getting this right is essential for a realistic portrayal. However, one element that is not necessary except in these considerations is to keep the size of the ear the same as the eye, hence the correct answer is A. Additionally, it's crucial to understand that having the eyes located at the very center of the head is also not generally true.
Contrast in value helps to create a dynamic composition in painting by providing variations that can make elements stand out. Specifically, contrast helps to create a realistic image that can stand out against the background, which corresponds to answer D.
Keeping facial features in proportion essentially means ensuring that all parts of the face have a logical relation to each other; it involves more than merely using a ruler or positioning features based on viewpoint. The correct answer in this case is D.