The correct answer is mouth. While each answer choice has a part in the digestion process, carbohydrate digestion begins in the mouth, which provides enzymes required for starch (amylase) and sugar (lactase, maltase, sucrase) digestion.

Carbohydrate digestion (starches and sugars) begins in the mouth with the enzyme salivary amylase. Amylase, which is also found in the small intestine, is responsible for breaking apart starches. Sugars are also digested (broken down) by three major enzymes located in the mouth and small intestine (lactase, maltase, sucrase). The carbohydrates are broken down along the gastrointestinal tract (mouth, pharynx, esophagus, stomach) and are absorbed while moving through the small intestine.

Styloglossus: elevates and retracts tongue

Hyoglossus: depresses and retracts the tongue

Palatoglossus: elevates the tongue

Vertical muscles: this is an intrinsic muscle of the tongue

"Deglutition" is the scientific term for swallowing. In contrast, the scientific term for chewing is "mastication." The scientific term for urination is "micturition." Lastly, "voiding" is also another word for urination. 

Proteins need pepsin in order to begin digestion, and that enzyme is only produced in the stomach — not the mouth.

Pepsinogen is an inactive enzyme that is released into the stomach lumen by chief cells. Parietal cells are responsible for secreting hydrochloric acid. This acid will cleave the pepsinogen and make it an active enzyme, pepsin, which can then cleave peptide bonds and begin protein digestion.

Goblet cells secrete mucus to protect the epithelium of the stomach from the acid in the lumen. G cells secrete gastrin, which promotes the secretion of hydrochloric acid and pepsinogen.

Stratified squamous epithelium protects tissues in areas that are prone to abrasion. This lining consists of many layers and is typically located on the areas near/associated with the mouth, excretory system, and the esophagus. The stomach does not have a stratified squamous epithelial lining, as it instead uses simple columnar epithelium.

The stomach stores ingested food, denatures proteins, initiates protein digestion, and mechanically breaks down food. The absorption of triglycerides, or lipids, takes place in the small intestine.

Liquids exit the stomach more quickly than solids, and isotonic solutions exits more quickly than hypertonic or hypotonic solutions. As such, isotonic liquids will exit the stomach most quickly during gastric emptying.

Solids exit the stomach in the general order of carbohydrates, followed by proteins, followed by fats.

Goblet cells secrete mucous, which protects the stomach lining. Chief cells secrete pepsinogen, which must be converted to pepsin by acid in order to carry out its function of breaking down proteins. G-cells secrete gastrin, increase secretion from the other cells, and induce muscle contractions. Parietal cells secrete both  and intrinsic factor.

Since the stomach releases  ions into the stomach, it releases bicarbonate into its venous supply, which can feed into the pancreas. This allows the pancreas to release the bicarbonate into the lumen to help neutralize the stomach acid. Remember, the venous blood leaving the pancreas is acidic in nature and can help provide the stomach with a supply of  ions for its functionality.