Option 1: Johannesburg is not the largest metals trading centre. It is a major financial centre in Africa, but it does not have the same level of activity in metals trading compared to other options.
Option 2: London is a major global financial centre and has a significant presence in metals trading. However, it is not the largest metals trading centre.
Option 3: New York is the correct answer. It is recognized as the largest metals trading centre in the world due to the presence of the London Metal Exchange (LME), where a significant amount of global metals trading takes place.
Option 4: Singapore is a growing financial centre and has become an important player in commodities trading. However, it is not the largest metals trading centre.
Therefore, the correct answer is option 3, New York. It is home to the largest metals trading centre due to the presence of the LME.