The answer is water.
That is when two monosaccharides link to form a disaccharide, the by-product is water.
Here glucose and fructose are monosaccharide that combines to form a disaccharide that is sucrose and water formed as a byproduct that can be seen in the image attached.
So Water is the byproduct that is formed, when two monosaccharides link to form a disaccharide.
