The correct answer is option 2: Phytochrome.

Phytochrome is a pigment found in plants that plays a crucial role in perceiving photoperiodic stimuli. Photoperiodic stimuli refer to changes in day length or periods of daylight that plants use to regulate various developmental processes such as flowering, seed germination, and other morphogenetic phenomena.

Cytochrome, on the other hand, is a type of protein involved in electron transfer during cellular respiration and photosynthesis. While it has important roles in plant metabolism, it is not directly involved in perception of photoperiodic stimuli.

Phycobilins and phycoerythrin are types of pigments found in algae and cyanobacteria. They are involved in light absorption during photosynthesis in these organisms, but they do not play a direct role in photoperiodic perception in plants.

In conclusion, phytochrome is the pigment in plants that is responsible for perceiving photoperiodic stimuli and controlling important developmental processes.