Sea water : Salty :: ______: Soap
The correct answer is option D: Both (a) and (b).

Explanation:
Analogy: An analogy is a comparison between two things that are similar in some way but otherwise different. It helps us understand the relationship between two concepts by highlighting their similarities.

Sea water : Salty :: ______: Soap
In this analogy, we are comparing the relationship between sea water and salty to an unknown relationship between something and soap.

Sea water : Salty
Sea water is known to be salty. Salinity refers to the saltiness or concentration of salt in water. Sea water contains various dissolved salts, such as sodium chloride, which gives it its characteristic salty taste.

Soap:
Soap is a substance that is used for cleaning and washing. It is typically made by combining fats or oils with an alkali, such as sodium hydroxide or potassium hydroxide. Soap molecules have a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. This unique structure allows soap to interact with both water and oil, making it an effective cleanser.

Both (a) and (b):
The correct answer is option D: Both (a) and (b). Both bitter and slippery are characteristics or properties associated with soap.

Bitter:
Soap is often associated with a bitter taste. Some soaps, especially those that contain certain additives or chemicals, may have a bitter flavor if accidentally consumed. This is why it is important to avoid ingesting soap and to rinse thoroughly after use.

Slippery:
Soap is also known to be slippery. When soap comes into contact with water, it creates a lather or foam that reduces friction. This slippery texture allows soap to glide smoothly over surfaces and helps in the removal of dirt and oils from the skin or other objects.

Conclusion:
In conclusion, the analogy "Sea water : Salty :: ______: Soap" can be completed with the answer option D: Both (a) and (b). Soap is associated with both a bitter taste and a slippery texture, making it similar to sea water being associated with saltiness.