At $25^{\circ} C$, the concentration of $H ^{+}$ions in $1.00 \times 10^{-3} M$ aqueous solution of a weak monobasic acid having acid dissociation constant $\left(K_a\right)=4.00 \times 10^{-11}$ is $X \times 10^{-7} M$. The value of $X$ is $...$ .
Use: Ionic product of water $\left(K_w\right)=1.00 \times 10^{-14}$ at $25^{\circ} C$

• A
  $3.23$ or $3.24$
• B
  $1.23$ or $1.24$
• C
  $2.23$ or $2.24$
• D
  $1.50$ or $1.76$