The correct answer is a) filtration.
Explanation: The digestive system is primarily responsible for processing food to extract energy and nutrients. Its main functions include:
- Ingestion: Taking food into the body.
- Mechanical Processing: Physically breaking down food into smaller pieces, which occurs during chewing.
- Absorption: The process of taking nutrients from the digestive tract into the bloodstream.
- Compaction: The process of consolidating indigestible materials into feces.
Filtration, however, is not a function of the digestive system. This process is more associated with the kidneys and the urinary system, which filter blood to remove waste products.