Mineral salts can be absorbed into roots by

Analyze the question: It asks how mineral salts (dissolved ions) enter plant roots.

Explain the concept:

• Osmosis is the movement of water across a semi-permeable membrane from a region of high water potential to low water potential. It applies to water, not to mineral ions.
• Diffusion is the movement of particles from high to low concentration. It requires no energy. Mineral ions can enter this way when the soil concentration is higher than the concentration in the root cells.
• Active transport is the movement of ions against a concentration gradient (from low to high) using energy (ATP) and carrier proteins. This is the main way roots absorb minerals, because the soil water is often more dilute in minerals than the root cells.
• Imbibition is the absorption of water by solids such as seeds or colloids (e.g. a dry seed swelling). It is not a mechanism for mineral uptake.

Solution:

• A (Osmosis only): wrong, because osmosis deals with water, not salts.
• B (Osmosis and diffusion): wrong, because osmosis is not involved in ion uptake.
• C (Diffusion and active transport): correct. Ions move in by diffusion when the gradient favours it, and by active transport when they must move against the gradient.
• D (Imbibition only): wrong, because imbibition concerns water absorption by dry materials.

Answer: Option C.

Tip: Remember "Water = Osmosis, Minerals = Diffusion and Active transport." If a question asks about salts or ions, rule out osmosis and imbibition straight away.