The most stable oxidation state of boron is +3 (option C). Let's understand why this is the case:

1. Introduction:
Boron is a chemical element with atomic number 5 and symbol B. It is a metalloid and is commonly found in compounds such as borates. Boron can exhibit a range of oxidation states, but the most stable and commonly observed oxidation state is +3.

2. Electronic configuration:
The electronic configuration of boron is 1s² 2s² 2p¹. Boron has three valence electrons in its outermost shell, and it tends to lose these electrons to achieve a stable electronic configuration.

3. Stability of +3 oxidation state:
When boron loses its three valence electrons, it forms a +3 oxidation state. This oxidation state is stable due to the following reasons:

a) Octet rule: By losing three electrons, boron achieves a stable electron configuration similar to that of a noble gas (helium). This stable configuration follows the octet rule, which states that atoms tend to gain, lose, or share electrons to achieve a stable configuration with eight valence electrons.

b) Electronegativity: Boron is less electronegative compared to many other elements. This means it has a lower tendency to gain electrons and a higher tendency to lose electrons. Hence, it is more favorable for boron to lose three electrons and attain the +3 oxidation state.

c) Stability of compounds: Boron forms numerous stable compounds in its +3 oxidation state. For example, boron trichloride (BCl₃), boron trifluoride (BF₃), and boron trioxide (B₂O₃) are all stable compounds that contain boron in its +3 oxidation state.

4. Other oxidation states:
Although the +3 oxidation state is the most stable, boron can also exhibit other oxidation states such as +1 and +2. However, these oxidation states are relatively less common and less stable compared to the +3 oxidation state.

In conclusion, the most stable oxidation state of boron is +3. This is due to the stability of the electron configuration, the lower electronegativity of boron, and the formation of stable compounds in this oxidation state.