Here's why:

* Glucose is a neutral molecule. This means the overall charge of the molecule is zero.

* Carbon forms four bonds in glucose. These bonds are with hydrogen, oxygen, and other carbon atoms.

* Hydrogen typically has an oxidation state of +1.

* Oxygen typically has an oxidation state of -2.

To calculate the oxidation state of carbon, we can use the following equation:

Oxidation state of carbon + (number of C-H bonds) + (number of C-O bonds) = 0

In glucose, there are:

* 6 carbon atoms

* 12 hydrogen atoms

* 6 oxygen atoms

This gives us the following:

* 6C + 12H + 6O = 0

Since each hydrogen has a +1 oxidation state and each oxygen has a -2 oxidation state, we can substitute these values:

* 6C + 12(+1) + 6(-2) = 0

* 6C + 12 - 12 = 0

* 6C = 0

* C = 0

Therefore, the oxidation state of carbon in glucose is 0.