Final answer:
V-type proton pumps in the endosome membrane are responsible for the drop in pH that marks the maturation of endosomes and the conversion to lysosomes.
Step-by-step explanation:
The drop in pH that marks the maturation of endosomes and the conversion to lysosomes is likely due to v-type proton pumps in the endosome membrane. These pumps actively transport protons into the endosome, making the interior more acidic. The acid hydrolases, soluble proteins, and lysosomal-directed tags on membrane proteins all play important roles in the function of lysosomes, but they are not primarily responsible for the pH change.