Hey,
Now you have an isosceles trinagle ABC.
Now the angles ACB and ABC are equal. If one of the angle is given to be 30 degrees. There are two possibilities:
Case 1: \(\angle ACB = \angle ACB =30\). In this case \(\angle BAC = 180 -30 -30= 120.\)
Case 2: \(\angle BAC = 30\). In this case \(\angle ACB = \angle ACB =\frac{180 -30}{2}=75\)
These are the only two possible cases. Since case 1 has the higer value of the maximum angle hence option D is correct!
Attachment:
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PS: You figured the solution correctly. When they say highest possible value, they mean of all the possible cases the maximum possible is the answer they expect.