To solve this problem, we need to understand the role of different cell organelles in regulating osmotic expansion.$\newline$Osmotic expansion refers to the increase in cell volume due to water intake when a cell is placed in a hypotonic solution, such as pure water.$\newline$Let's analyze the options:$\newline \bullet$Mitochondria:$\newline$ - Mitochondria are the powerhouse of the cell, responsible for energy production through ATP synthesis.$\newline$ - They do not play a direct role in regulating osmotic expansion.$\newline \bullet$Vacuoles:$\newline$ - Vacuoles are membrane-bound organelles that store nutrients, waste products, and help maintain turgor pressure in plant cells.$\newline$ - They are crucial for osmoregulation, as they can store or release water to balance the cell's internal environment.$\newline$ - In plant cells, vacuoles are primarily responsible for regulating osmotic expansion by adjusting the water content.$\newline \bullet$Plastids:$\newline$ - Plastids are involved in photosynthesis (chloroplasts) and storage of pigments and starch.$\newline$ - They do not directly regulate osmotic expansion.$\newline \bullet$Ribosomes:$\newline$ - Ribosomes are involved in protein synthesis.$\newline$ - They do not have a role in osmoregulation or osmotic expansion.$\newline$From this analysis, we can conclude that vacuoles are the organelles chiefly responsible for regulating the osmotic expansion of a cell kept in water.$\newline$Therefore, the correct option is Option 2: Vacuoles.