The right answer is c. active transport.
In biology, active transport refers to the passage of an ion or a molecule across a membrane against its concentration gradient. If the process uses chemical energy produced, for example, by the hydrolysis of a nucleotide triphosphate such as adenosine triphosphate, it is called primary active transport. Secondary active transport involves the use of an electrochemical gradient. Active transport is opposed to passive transport, which does not use energy.
Specialized transmembrane proteins recognize the molecules / ions and allow them to (or in the case of secondary transport, expend energy to force them to) cross the membrane, whereas without these proteins this would be impossible, either because of the presence of the lipid bilayer impervious to them, either because it is a movement against a concentration gradient. In the latter case, known as primary active transport, the protein acts as a pump that uses chemical energy, usually ATP. In other cases, the energy comes from the use of an electrochemical gradient or transport is provided for example by symports or antiports.
When particles are to be transferred from a low-concentration compartment to a concentrated compartment (ie against the concentration gradient), an active transporter becomes necessary to effect the transfer. Transport molecules are specific to the molecules they carry (for example for glucose).
