The correct answer is A. when the shock wave, associated with the gusty solar wind, compresses the magnetosphere.
The magnetosphere is the region of space around Earth that is dominated by Earth’s magnetic field. The solar wind is a stream of charged particles from the Sun. When the solar wind is gusty, it can compress the magnetosphere. This compression can cause the magnetic field to increase.
Option B is incorrect because a large decrease in field intensity would cause a magnetic storm, not an initial increase in magnetic field. Option C is incorrect because the gust wind itself does not engulf the magnetosphere. The magnetosphere is much larger than the gust wind. Option D is incorrect because the answer is A.