Correct choice is (a) per sq m
The explanation: To ensure your finished oil or acrylic painting to stay looking its best then adding the right varnish in the right way will be a sound investment. Varnish protects the painting from dirt and dust and evens out the painting’s final appearance, making it all equally glossy or matt.
Over the years dirt and dust will stick to the varnish rather than the painting and when the time is right, the varnish itself can be removed and the painting re-varnished to look as good as new.