**Core Concept:** Intracellular fluid (ICF) is the fluid contained within the cells of an organism. It is composed of water, electrolytes, and various organic compounds, including glucose, amino acids, and fatty acids. The primary component of ICF is the water, which constitutes about 90% of ICF volume.

**Why the Correct Answer is Right:** The correct answer, **D:** Water, is right because water makes up the majority of intracellular fluid volume, accounting for around 90% of ICF. This is due to the semi-permeable cell membrane, which prevents large molecules and ions from passing freely between the extracellular and intracellular spaces. The remaining space is occupied by the smaller molecules and ions, such as potassium, sodium, chloride, and calcium ions, as well as organic molecules like glucose, amino acids, and fatty acids.

**Why Each Wrong Option is Incorrect:**

A. Glucose: While glucose is present in ICF, it is not the main component.
B. Amino Acids: Amino acids are constituents of ICF, but they are not the primary component.
C. Fatty Acids: Fatty acids are also part of ICF, but they are not the main component.

**Clinical Pearl / High-Yield Fact:** Understanding the composition of intracellular fluid is essential for understanding cellular processes such as osmoregulation, ionic balance, and nutrient transport across cell membranes. This knowledge aids in diagnosing and treating various medical conditions involving fluid and electrolyte imbalances, as well as understanding the role of medications like diuretics and vasopressors in altering ICF and ECF balance.