Final answer:
The most popular potted plant in December is the Poinsettia, and plants like the Christmas cactus bloom in response to environmental cues through photoperiodism.
Step-by-step explanation:
The most popular potted plant in December is typically the Poinsettia (option a). Known for its bright red and green foliage, the Poinsettia is closely associated with Christmas celebrations. However, the student's question seems to be indirectly referencing the Christmas cactus, which is known to bloom during the Christmas season, despite plants not being able to read a calendar.
Plants like the Christmas cactus know it's time to bloom through environmental cues such as temperature and light. As the days get shorter and the temperature drops during the winter months, these plants receive signals to initiate their blooming process. This phenomenon is called photoperiodism, where plants respond to the length of day and night to time their growth and reproductive cycles. Therefore, the Christmas cactus often blooms around Christmas because the environmental conditions typical of that time of year are ideal for triggering its flowering cycle.