Final answer:
The marine biome includes both saltwater and freshwater ecosystems, with estuaries serving as the transitional zone where these two types of water meet. Option d.
Step-by-step explanation:
A biome that encompasses both saltwater and freshwater ecosystems is the marine biome. Marine biomes consist of oceans, coral reefs, and estuaries, which are coastal areas where salt water from the sea mixes with fresh water from rivers and streams. In contrast to freshwater biomes, which have a salt concentration of less than 1% and include lakes, ponds, and rivers, marine biomes have a higher salt concentration and are characteristic of larger bodies of water like the ocean. The marine biome includes both saltwater and freshwater ecosystems, with estuaries serving as the transitional zone where these two types of water meet. Option d.