Final answer:
The switch from endosome to lysosome is known as maturation, beginning with lysosomal protein synthesis in the RER, followed by packaging, Golgi modification, and distribution resulting in a mature lysosome.
Step-by-step explanation:
The process by which an endosome transitions to become a lysosome is known as maturation. This involves a series of changes including acidification, the loss of the clathrin coat from coated vesicles, and the sorting of vesicle contents. This maturation process continues as the vesicle merges with a late endosome to form what is more properly termed a pre-lysosomal compartment or an endolysosome. Then, the mature endolysosome transforms into a functional lysosome capable of digesting its contents through the activity of hydrolytic enzymes.
The formation of these organelles begins in the rough endoplasmic reticulum (RER), where the synthesis of lysosomal proteins occurs. These proteins are subsequently packaged into vesicles, modified in the Golgi apparatus, and then distributed to form a mature lysosome.
Therefore, the correct answer to the question, 'What switch is gradually made from endosome to lysosome?' is B. Maturation; Early endosome.