Assertion : Boron is hard element.

Reason : Boron has unusually high melting point.

The correct option is: "A is correct. R is correct R does not explains A."

Let us analyze the assertion and the reason step-by-step to understand why this option is correct.

1. Analysis of Assertion (A):
The assertion states that "Boron is a hard element".
Boron exists in several allotropic forms. The most common crystalline structure of boron consists of extremely strong, three-dimensional covalent networks made of icosahedral B₁₂ units. Because of these exceptionally strong covalent bonds throughout its crystal lattice, boron is extremely hard—ranking just below diamond on the Mohs hardness scale. Thus, the Assertion (A) is correct.

2. Analysis of Reason (R):
The reason states that "Boron has an unusually high melting point".
Due to the same strong, closely packed three-dimensional polymeric covalent network structure, a vast amount of thermal energy is required to break the bonds and melt the solid. Consequently, boron possesses a very high melting point (around $2350K^{\circ }$). Thus, the Reason (R) is also correct.

3. Link between Assertion and Reason:
Although both statements are factually correct, the Reason (R) does not explain the Assertion (A). The hardness of boron is a measure of its resistance to surface deformation or scratching, which is due to its strong covalent bonding and structure. The high melting point is a thermal property, also resulting from the strength of the lattice, but one is not the explanation/cause of the other. They are independent consequences of the strong covalent polymeric network of boron. Therefore, the reason does not explain why boron is hard.