All but one is a monomer of a disaccharide. that is

  1) A monomer isn't always a monosaccharide, but all monosaccharides are monomers. 

2) A polymer can be a chain of monomers, or repeating units. The basic principal behind polymers is that it is composed of many repeating units. 

3) Taking the definition from (2), my opinion would be a dissacharide would only be a polymer if it were constructed to two of the same monomer, e.g. maltose (2 glucose subunits). So TECHNICALLY, it is a polymer, since its composed of two identical ("repeating") subunits. 

In a disaccharide composed of two different monosaccharides (e.g. sucrose, from 1 glucose and 1 fructose subunit), there is no pattern or repetition, so I don't think it would be a polymer, and therefore not a polysaccharide. 

4) Typically, polysaccharides are in the order or 100s or 1000s of linked monosaccharides.

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