Identify Incorrect Statement Regarding Carbohydrates and Monosaccharides

To determine the incorrect statement regarding carbohydrates, let us analyze each of the given options:

1. Option A: "All monosaccharides are reducing sugars."
   Monosaccharides (whether aldoses or ketoses) possess a free carbonyl group (in open-chain form) or a hemiacetal/hemiketal group (in cyclic form) capable of acting as a reducing agent towards Tollen's reagent and Fehling's solution. Thus, all monosaccharides are reducing sugars. This statement is correct.

2. Option B: "The monosaccharide units obtained from hydrolysis of oligosaccharides are always the same."
   Oligosaccharides are carbohydrates that yield $2$ to $10$ monosaccharide units upon hydrolysis. These constituent monosaccharide units can be either identical or different:

   • Hydrolysis of maltose yields two identical units of $\text{D-glucose}$: $\text{Maltose} + \text{H}_2\text{O} \xrightarrow{\text{Hydrolysis}} 2\,\text{D-Glucose}$
   • Hydrolysis of sucrose yields two different monosaccharide units ($\text{D-glucose}$ and $\text{D-fructose}$): $\text{Sucrose} + \text{H}_2\text{O} \xrightarrow{\text{Hydrolysis}} \text{D-Glucose} + \text{D-Fructose}$ Since the monosaccharide units are not always the same, this statement is incorrect.

3. Option C: "Starch and cellulose are typical examples of polysaccharides, which are very high molecular weight compounds of more than ten monosaccharide units."
   Polysaccharides are polymers consisting of a large number (typically $>10$) of monosaccharide units linked by glycosidic bonds. Starch and cellulose are well-known naturally occurring polysaccharides with high molecular weights. This statement is correct.

4. Option D: "Open chain and cyclic structures co-exist at equilibrium that are responsible for certain properties as in the case of $\text{D-(+)}$-glucose."
   In aqueous solution, $\text{D-(+)}$-glucose undergoes mutarotation, reaching a dynamic equilibrium between its open-chain structure and its cyclic $\alpha$- and $\beta$-pyranose forms. This statement is correct.

Hence, the incorrect statement is given by Option B.