The correct answer is $\boxed{\text{C}}$.
The root mean square (rms) value of a sinusoidal AC current is equal to its value at an angle of 45 degrees. This is because the rms value is the square root of the average of the squared values of the current over a cycle. The average of the squared values of the current over a cycle is equal to the average of the values of the current over a cycle, which is equal to the value of the current at an angle of 45 degrees.
Option A is incorrect because the rms value of a sinusoidal AC current is not equal to its value at an angle of 90 degrees. The value of the current at an angle of 90 degrees is zero.
Option B is incorrect because the rms value of a sinusoidal AC current is not equal to its value at an angle of 60 degrees. The value of the current at an angle of 60 degrees is $\frac{\sqrt{3}}{2}$ times the rms value.
Option D is incorrect because the rms value of a sinusoidal AC current is not equal to its value at an angle of 30 degrees. The value of the current at an angle of 30 degrees is $\frac{\sqrt{2}}{2}$ times the rms value.