To show a contrast between Mount Everest and Mount Kea, the best option to place in the blank is "despite."
So the sentence might read: "Despite their notoriety as significant mountains, Mount Everest and Mount Kea differ in elevation and geological characteristics."
The other options do not effectively convey a contrast:
- "In addition to" implies similarity or additional information.
- "In similar fashion to" implies similarity.
- "Because of" indicates a causal relationship rather than a contrast.