Assertion: Out of 40,000 km of railways in Africa continent, South Africa alone account for 18,000 km.
Reasoning: South Africa is very rich in minerals like copper, diamond and Gold.

Both Assertion (A) and reasoning (R) are correct and R is the correct explanation of A.

The correct answer is option 1: Both Assertion (A) and reasoning (R) are correct and R is the correct explanation of A.

Assertion: Out of 40,000 km of railways in Africa continent, South Africa alone account for 18,000 km. Correct because South Africa has one of the most developed railway networks in Africa. 
Reasoning: South Africa is very rich in minerals like copper, diamond and Gold. Correct. 

Does R explains A? Yes. The abundance of valuable minerals in South Africa increased the need to transport heavy mineral resources from mines to industries and ports for export. Railways are the most suitable and economical means for carrying bulky minerals over long distances. Therefore, to support mining and trade activities, a large railway network developed in South Africa. Hence, the rich mineral resources are a major reason why South Africa has such an extensive railway system compared to other African countries.

NCERT: " Africa continent, despite being the second largest, has only 40,000 km of railways with South Africa alone accounting for 18,000 km due to the concentration of gold, diamond and copper mining activities."