Final answer:
Serine/cysteine protease inhibitors are involved in regulating programmed cell death in plants by preventing the degradation of certain proteins.
Step-by-step explanation:
The effect of serine/cysteine protease inhibitor is mainly associated with regulating various physiological processes, including programmed cell death (PCD) in plants. When protease inhibitors are present, they can prevent proteases from degrading specific proteins, which in turn can lead to the inhibition of PCD. According to the reference from Solomon et al. (1999), the involvement of cysteine proteases and protease inhibitor genes play a significant part in the regulation of programmed cell death in plants. Thus, the correct answer to the question "What is the effect of serine/cysteine protease inhibitor?" is C. Inhibits programmed cell death.