D

The coatings on raincoats are specifically designed to repel water, thus keeping the wearer dry. The key property responsible for this is the contact angle of water on the surface of the coating.

Step 1: When water hits a surface, it can either spread out (wetting) or bead up (non-wetting) depending on the surface properties.

Step 2: A surface is considered waterproof if the contact angle is relatively high (usually greater than 90 degrees), causing the water to form droplets rather than spreading out. This high contact angle is achieved through the materials used in the coating, which can alter the surface tension of the water.

Step 3: When the angle of contact decreases, the water tends to spread out, which is not desirable for waterproofing. Instead, we typically aim for an increased angle of contact to ensure that water beads up and does not penetrate through the fabric.

Conclusion: Therefore, the answer is option D: Angle of contact decreases.