Based on the scenario described, the most supported claim is:
A. The moon was passing between the sun and Earth.
The scenario resembles a solar eclipse, where the moon moves between the Earth and the sun, causing a temporary decrease in sunlight. The other options do not logically explain a sudden, temporary decrease in light intensity in the absence of any clouds. An airplane would not block sunlight for an extended period, variations in the sun's output do not happen on such short timescales, and a sudden change in temperature in the atmosphere is not likely to cause a immediate dramatic decrease in light intensity.