The correct answer is C. since.
The word “since” is used to indicate a point in time in the past from which something continues to happen. In this case, the speaker is saying that nobody has come to see them since they bought the dogs. This means that the lack of visitors started at the time they bought the dogs and has continued ever since.
The other options are not as accurate. The word “when” is used to indicate a specific point in time. In this case, the speaker is not saying that nobody came to see them at a specific point in time, but rather that nobody has come to see them since they bought the dogs. The word “till” is used to indicate a point in time in the future. In this case, the speaker is not saying that nobody will come to see them until they get rid of the dogs, but rather that nobody has come to see them since they bought the dogs. The word “for” is used to indicate a period of time. In this case, the speaker is not saying that nobody has come to see them for a specific period of time, but rather that nobody has come to see them since they bought the dogs.