Step 1: Understanding the Question
This is a geography/general knowledge question asking to identify the major mineral deposit found in the Kalgoorlie and Coolgardie regions of Western Australia.
Step 2: Recalling Geographical Facts
The state of Western Australia is a major global producer of minerals.
The towns of Kalgoorlie and Coolgardie are historic centers of the Western Australian gold rushes of the late 19th century.
The area around Kalgoorlie, particularly a stretch known as the "Golden Mile," is considered one of the richest goldfields in the world and continues to be a major gold-producing region.
Step 3: Evaluating the Options
(A) Gold: This is the correct answer. Kalgoorlie is synonymous with gold mining in Australia.
(B) Diamond: Major diamond mining in Australia is associated with the Argyle mine, also in Western Australia, but not with Kalgoorlie.
(C) Platinum \& (D) Silver: While these may be found in Australia, Kalgoorlie's fame and largest deposits are overwhelmingly associated with gold.
Step 4: Final Answer
The Kalgoorlie and Coolgardie areas are world-renowned for their vast gold deposits. Therefore, option (A) is the correct answer.