C给的解释是 'Although' typically introduces a subordinate clause, which has a subject and a verb, but here there is no subject and 'sighted' is not a complete verb phrase.
08 的Prep语法笔记44页上讲到“状语从句的省略”,其中第二点是“从句的谓语必须是be.....从句的主语和谓语要么同省,要么同留“。而从这道题的题干所给的原句可以看出,从句的谓语动词是”have been“(are visible as dark spots on the surface of the sun but have never been sighted on),因此也就不符合这个【状语从句省略】的规则。