What is the focal distance of any point $\mathrm{P}\left(x_1, y_1\right)$ on the parabola $y^2=4 a x ?$

• A

  $x_1+y_1$
• B

  $x_1 y_1$
• C

  $a x_1$
• Correct answer
  D

  $a+x_1$

Hence, the correct answer is option: 'D'.