(A) works fine here. The sentence structure here is:
"Sunspots are visible as [x] but have never been sighted on [Y]."
Here it doesn't make sense to compare "dark spots on" to "sighted on" simply because it doesn't fit into the framework appropriate for this particular sentence structure.
"Sunspots are visible as [x] but have never been sighted on [Y]."
Here it doesn't make sense to compare "dark spots on" to "sighted on" simply because it doesn't fit into the framework appropriate for this particular sentence structure.