Final answer:
The East African Rift Valley (C) is the geological feature in northeast Africa that has failed to open into a new ocean basin, unlike the Red Sea and the Gulf of Aden which have already become oceanic basins.
Step-by-step explanation:
The geological feature that has failed to open into another new ocean basin in northeast Africa is the East African Rift Valley (C). The Great Rift Valley is a continuous geographic trench that currently is in the process of splitting the African Plate into the Somalian Plate and the Nubian Plate. However, unlike the Red Sea and the Gulf of Aden, which are already oceanic basins, the East African Rift is an active continental rift zone that has yet to develop into a full-fledged oceanic basin.
The Red Sea and the Gulf of Aden are examples where rifting has already led to the formation of new ocean basins, indicating a more advanced stage of rifting. Meanwhile, the East African Rift is still in a stage of creating a rift valley which may eventually lead to the creation of a new ocean if the rifting continues. The Triple Junction refers to the point where three rifts meet, which includes the Red Sea Rift, the Gulf of Aden Rift, and the East African Rift.