If $\frac{a x+5}{\left(x^2+b\right)(x+3)}=\frac{x+21}{12\left(x^2+b\right)}+\frac{c}{12(x+3)}$, then $b^2=$

• Correct answer
  A

  $a^3-c$
• B

  $a^2+c$
• C

  $a-c$
• D

  $a+c$

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