From the given cities, state whether it is a megacity (metropolitan area of over 10 million people), a national capital, or if they are both a megacity and capital or neither a megacity or capital.
Oops, confused Istanbul with Islamabad. However, Singapore is not the officially designated capital of Singapore, it being the only city in the country. I would recommend replacing this question. Would also suggest replacing Johannesburg, as Pretoria is included in the urban population of Johannesburg.
It doesn't really change the fact that Singapore city is the capital of Singapore and does not have more than 10 million people so there is only one answer it could be. For Johannesburg, as I mentioned in the description, it is based on urban agglomeration which, yes, Pretoria is included. According to Citypopulation.de and Demographia, Johannesburg is a megacity, having 14 million people.