The number of moles of oxygen in one litre of air containing 21% oxygen by volume, under standard conditions are:

The correct option is A $0.0093\text{moles}$
1 litre of air contains 21% of oxygen.
Total amount of oxygen in $1\text{L}=0.21\times 1\text{L}=0.21\text{L}$
At STP, $22.4\text{L}$ volume is occupied by 1 mole of oxygen
So, $0.21\text{L}$ volume is occupied by $=\frac{1\times 0.21}{22.4}=0.0093\text{moles}$ of oxygen