To solve this question, we need to understand the changes that occur in the composition of the fetal body as gestational age progresses.

Fetal development involves various changes in body composition, which includes elements like water, fat, calcium, and phosphorus. Here's a detailed explanation:

• Water: During early gestation, the fetal body contains a higher percentage of water. As the fetus grows, water content gradually decreases as other components like fat and tissues develop. Therefore, the water content in the fetal body decreases with the increase in gestational age. This makes water the correct answer.
• Calcium: Calcium content increases as the fetus develops. It is crucial for the formation of bones and teeth, which become more defined towards the later stages of pregnancy.
• Phosphorus: This element also increases because it is needed for bone and tissue development, similar to calcium.
• Fat: Fat accumulation increases as the fetus requires more energy for growth, and it also serves as insulation for temperature regulation.

Therefore, among the given options, water is the component that gradually reduces with the increase in gestational age.

Based on this analysis, the correct answer is water.