Which one of "television or printed copies of periodicals or books" has become (or will become) vulnerable to substitution?
Both of these products could be vulnerable to substitution, but printed materials are more vulnerable and have already been affected by substitution.
The main reason for this is the internet. With the availability of both e-books and websites with a great deal of information, printed materials are becoming somewhat endangered. So far, television has not faced the same level of competition from online substitutes. It is not as easy to find and download television shows in a timely manner. It is also not as easy to find a substitute for a given TV show as it is to find an alternative to a given magazine. This means that people will be less likely to try to find online substitutes for television than for books and other printed materials.