Gold was discovered in Australia in 1651.

The statement is a declarative sentence. To form a question from it, we need to use an auxiliary verb at the beginning. Option 'd' correctly forms a question by using 'Was Gold discovered...' which is grammatically sound and directly asks about the statement's content. Options 'a', 'b', and 'c' have grammatical errors ('Did Gold was', 'Was Gold was', 'Were Gold was').