Let the points $A:\left(0,a\right), B:\left(-\mathrm{2,0}\right)$ and $C:\left(1,1\right)$ form an obtuse angled triangle (obtuse angled at angle $A$), then the complete set of values of $a$ is

• A

  $\left(-\mathrm{2,1}\right)$
• B

  $\left(-\mathrm{2,1}\right)-\left\{\frac{2}{3}\right\}$
• C

  $\left(-\mathrm{1,2}\right)$
• Correct answer
  D

  $\left(-\mathrm{1,2}\right)-\left\{\frac{2}{3}\right\}$

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