Agar space me sun nahi hota to abhi earth ka kya hota​

Option D The National Air and Space Museum is one of many free museums in Washington, D.C.

Explanation:

The reason is that the first letter of the name of place, person and city are always mentioned in capital. For example, if the name is George Bush then the first letter of first and last name must be capitalized according to grammar rules. As the proper name of the museum is "National Air and Space Museum", so that's the reason why the name of museum is capitalized.