Final answer:
Plants and algae both have chloroplasts (Option C) in common, which are essential for photosynthesis. The presence of vascular tissues such as phloem and xylem is unique to plants and not found in algae.
Step-by-step explanation:
The structures that plants and algae have in common are chloroplasts (Option C). Chloroplasts are essential for the process of photosynthesis, which both plants and algae use to convert light energy into chemical energy in the form of glucose. While phloem and xylem are part of the vascular tissue of plants and are responsible for the transport of substances within the plant, these structures are typically not found in algae, which do not have complex vascular systems like plants.