Assertion A states that steel is used in the construction of buildings and bridges, which is true. Steel is a widely used material for construction due to its high strength, durability, and resistance to corrosion.

Reason R states that steel is more elastic and has a higher elastic limit compared to other common construction materials like concrete, and this is why it is preferred in construction. This statement is also true. Steel's high elasticity and high elastic limit make it an ideal material for use in construction as it can withstand greater stress before it becomes permanently deformed.

Therefore, both Assertion A and Reason R are true statements. However, to determine the most appropriate answer, we need to see if Reason R explains Assertion A.

Reason R does indeed explain why steel is used in the construction of buildings and bridges. Its high elasticity and high elastic limit allow it to withstand greater stress than other common construction materials before becoming permanently deformed, making it a preferred material for construction.

So, the correct answer is Both A and R are correct, and R is the correct explanation of A.