Final answer:
The outer covering of an orange is known as the exocarp. It forms part of the pericarp, which also includes the mesocarp and endocarp, and is integral to the structure and function of the fruit, particularly for seed dispersal.
Step-by-step explanation:
Oranges have an exocarp, which is the outer covering of a fruit. The correct option to fill in the blank is 'exocarp'.
The exocarp, along with the mesocarp and endocarp, form the pericarp, which is the structure that encloses the seed and is a part of the fruit. Oranges, specifically navel oranges, have a characteristic 'navel' at the end opposite of the stem, which shows how an altered fruit development can occur. For example, two flowers produce close together can result in a single fruit with another smaller fruit inside it, as seen in the navel orange.
Fruits are structured to serve the primary function of seed dispersal. The pericarp matures as the seeds within the fruits mature, serving as a protective layer. The exocarp is usually what is seen as the fruit's skin, whereas the edible portion we commonly consume in the case of oranges is the mesocarp. The endocarp is typically the innermost layer that encloses the seed, and its texture can vary depending on the type of fruit.