An analogy of the form A : B works by identifying the exact relationship between A and B, then finding an option with the same relationship to the second word given. Here, Cobra is a specific type within the broader category Snake, so we need the option standing in that same 'specific : general' relationship with Greyhound.
The correct answer is Dog.
Another way to confirm this analogy is to check inclusion in reverse: does the broader category on the right actually include the specific item on the left as one of its members? 'Snake' includes 'Cobra' as one of its many members, so we look for the option that includes 'Greyhound' as one of its members in the same way.
Checking which category actually contains greyhound as a member, the way snake contains cobra, points to only one option.
Therefore, the correct answer is Dog.