21) A

1 – All DNA has a double-helix structure meaning mitochondria and human white blood cell contain double-helix DNA. So 1 is correct.

2 – Human cells do not contain a cell wall. Therefore, we can conclude that 2 is wrong.

3 – A mitochondria does not have a nucleus since the passage states that mitochondria may once have been an aerobic bacteria. We know bacteria do not have a nucleus. 3 is wrong

4 – Both must have a cell membrane since all cells have a cell membrane.

Statements 1 and 4 are correct.
So A is answer.