The correct answer is A. Matter expands on heating.
When matter is heated, the molecules or atoms that make up the matter move faster. This increased movement causes the matter to expand. For example, when you heat water, the water molecules move faster and the water expands. This is why water boils and turns into steam.
The other options are incorrect because they do not describe what happens to matter when it is heated. Option B, reduces, means to make something smaller. Option C, diminishes, means to become less in amount or degree. Option D, contracts, means to become smaller or narrower.
I hope this explanation is helpful. Please let me know if you have any other questions.