Boron does not act as reducing agent in the reaction
2B +3Mg ----------> Mg3B2 in this reaction Mg is oxidized by Mg2- .Hence B is act as oxidizing agent. B is itself reduced.
B reduces SiO2 to Si and CO2 to C .
(4B + 3SiO2 ---------> 3Si + 2B2O3 )
(4B + 3CO2 ----------> 3C + 2B2O3 )
Then the correct option is C .

**Answer:**

Boron (B) is a versatile element that can act as both an oxidizing agent and a reducing agent depending on the reaction conditions and the elements or compounds involved. In the given options, boron acts as a reducing agent in all the reactions except for option D) B + HNO3.

Let's examine each option in detail to understand why boron acts as a reducing agent in some cases and not in others:

**A) B + CO2:**
In this reaction, boron (B) can act as a reducing agent. Boron has a higher electronegativity than carbon, so it can donate electrons to carbon dioxide (CO2) and reduce it to carbon monoxide (CO). The reaction can be represented as follows:
B + CO2 → B2O3 + CO

**B) B + SiO2:**
In this reaction, boron (B) can also act as a reducing agent. Boron has a higher electronegativity than silicon, so it can donate electrons to silicon dioxide (SiO2) and reduce it to silicon (Si). The reaction can be represented as follows:
B + SiO2 → B2O3 + Si

**C) B + Mg:**
In this reaction, boron (B) can act as a reducing agent. Boron has a higher electronegativity than magnesium, so it can donate electrons to magnesium (Mg) and reduce it to boride (B2-). The reaction can be represented as follows:
B + Mg → MgB2

**D) B + HNO3:**
In this reaction, boron (B) does not act as a reducing agent. Nitric acid (HNO3) is a strong oxidizing agent and can oxidize boron. The reaction between boron and nitric acid can result in the formation of boron nitrate (B(NO3)3) and other nitrogen oxides. The reaction can be represented as follows:
B + HNO3 → B(NO3)3 + other nitrogen oxides

Therefore, the correct answer is option D) B + HNO3, where boron does not act as a reducing agent. In all the other options (A, B, and C), boron acts as a reducing agent by donating electrons to the elements or compounds involved in the reactions.