The correct answer is A) cirrocumulus. Cirrocumulus clouds are the type of cloud formation often referred to as a "mackerel sky" due to their small, rounded, and somewhat parallel cloud elements that resemble fish scales. This cloud type is generally associated with fair, stable weather conditions, as cirrocumulus clouds typically form at high altitudes and indicate the presence of high-pressure systems. The other answer choices are incorrect because stratus clouds (B) are low-lying, gray, and overcast, cumulonimbus clouds (C) are towering, thunderstorm-producing clouds, and nimbostratus clouds (D) are low-lying, dark, and often produce steady rain or snow.