The correct answer is D) evaporation of the water. A slow but continual loss of electrolyte level in a storage battery is typically caused by evaporation of the water content in the electrolyte solution. As the water evaporates over time, the electrolyte level will gradually decrease, even without any leaks or other issues. This is a normal occurrence and requires periodic refilling of the battery cells with distilled water to maintain the proper electrolyte level. The other answer choices are incorrect because: A) tight filler caps would not cause a gradual loss, B) higher specific gravity would not directly cause electrolyte loss, and C) a low charging rate may affect battery performance but would not specifically lead to electrolyte evaporation.