So, I'm not entirely sure why you think
Doesn't fit with option
c here. This just says that you can expect the grammar in question to be followed by something with a negative meaning; it's not for example suggesting that there has to be a negative conjugation involved.
心配 may literally mean worry, but it's being used here to express the fact that something is a bad idea - which seems negative enough to me.
However, it's kind of a moot point in this case anyway because the other two options don't really make sense.
When (someone is) going to buy an apartment, (I) worry about it after (they've) actually seen it. (?)
When (someone is) going to buy an apartment, (I) worry about it since (they've) actually seen it. (???)
When (one is) going to buy an apartment, (I) worry about it unless (they are) buying after having seen it.
It's pretty awkward to try and provide direct translations for these sentences because it's impossible to place all these implied subjects without more context, but if it helps you, here is a much more natural English translation for option
When shopping for apartments, it's not a good idea to buy something you haven't seen first.