A slipped disk is a condition ..... the intervertebral disk protrudes and presses on nerves.
2. which is
3. in which
1- What's wrong with THAT here?
2- I can't add IN add the rest of the sentence to get the correct usage of IN WHICH here.
That said, there are - however - a few iconoclasts who are not fond of 'in which'. They (all right, I) would prefer something like 'A slipped disk is a condition that involves....' For further navel-gazing and coat-trailing, see my blog. But if you're happier with short answers, stick with Mike!