Contour ploughing is an agricultural practice that involves ploughing along the contour lines of the land instead of up and down the slopes. This technique helps in soil conservation, which is the correct answer.

Option 1, stopping floods, is not directly related to contour ploughing. While contour ploughing can help prevent soil erosion, which may contribute to flooding, it is not the primary purpose of this technique.

Option 2, growing crops, is a general benefit of any agricultural practice including contour ploughing. However, it is not the specific purpose of contour ploughing.

Option 4, checking landslides, is incorrect. Contour ploughing can help prevent soil erosion, but it is not specifically aimed at preventing landslides.

Therefore, the correct answer is option 3, soil conservation.