Notice that rocks older than 11 million years are present west of the Juan de Fuca Ridge but not east of the ridge. What could be happening to the sea-floor rocks along line segment C-D that would explain why rocks older than 11 million years no longer exist on the seafloor east of the ridge

Due to volcanic eruptions.

Explanation:

The rocks older than 11 million years no longer exist on the seafloor east of the ridge because of the eruption that occurs on the seafloor which melted the 11 million years rocks. There is enormous volcanic eruption occurs from the ridge present on the seafloor which melted the rocks that are present on the seafloor so that's why we can say that the volcanic eruptions remove all the rocks of 11 years of age.