Aqueous solution of which of the following boron compounds will be strongly basic in nature?

We need to identify which boron compound gives a strongly basic aqueous solution.

Analyzing each compound shows that sodium borohydride (NaBH₄) hydrolyzes slowly in water and, although a strong reducing agent, does not produce a strongly basic solution, while lithium borohydride (LiBH₄) behaves similarly without imparting strong basicity. Diborane (B₂H₆) reacts with water to form boric acid (H₃BO₃), which is weakly acidic rather than basic. Borax (Na₂B₄O₇), when dissolved in water:

$$\text{Na}_2\text{B}_4\text{O}_7 + 7\text{H}_2\text{O} \rightarrow 2\text{NaOH} + 4\text{H}_3\text{BO}_3$$

is the salt of a strong base (NaOH) and a weak acid (H₃BO₃), so its aqueous solution is strongly basic. Therefore, Na₂B₄O₇ (borax) gives a strongly basic aqueous solution. The answer is Option D: Na₂B₄O₇.