You would use a tube that includes the actual tyre-size in its range. So a tube labeled 700x25-28c would work, as would a 700x28-32c. However, there are some differences you may wish to consider.

A 700x25-28c tube would be stretched thinner when inflated inside a 700x28c tyre compared to a 700x28-32c tube. This thinner tube would be more susceptible to flats. And it may even split at the seams easier. A 700x28-32c tube would be more durable in use, but may be more susceptible to installation damage such as pinches from tyre-levers or pinched between the tyre and rim.

Personally I always go with the largest tube that'll fit inside the tyre. I've gotten fewer flats that way even if it does take a little more care to install.