During the 1990s positive technological change in the production of chicken caused the price of chicken to fall. Holding everything else constant, how would this affect the market for pork (a substitute for chicken) ?

The market for pork would fall because a close substitute, chicken, is now less expensive, thus, people have less incentive to buy pork because they can buy chicken instead.

In other words, demand for pork fell because supply for chicken grew, chicken became cheaper, and part of the demand that was for pork before, transferred to chicken.