The correct answer is option 3. The gas collected from heating sulphur on a spatula is sulfur dioxide (SO2). Sulfur dioxide is a colorless gas and has a suffocating odor. When it comes into contact with moist litmus paper, it turns it to red. This indicates that sulfur dioxide is acidic in nature. The reaction between sulfur dioxide and water produces sulfurous acid, which is responsible for the red color change of the litmus paper.

Option 1 is incorrect because sulfur (S) does not combine with oxygen (O) to form SO. It forms SO2.

Option 2 is incorrect because sulfur dioxide (SO2) turns moist litmus paper to red, not to blue.

Option 4 is incorrect because dry litmus paper does not have any moisture to react with sulfur dioxide, so it does not change color.