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!



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.