Which of the following numbers is not a perfect square?

Let's analyze the given options:
* For option A: $\sqrt{146689}=383$, which is an integer. Thus, it is a perfect square.
* For option C: $\sqrt{103041}=321$, which is an integer. Thus, it is a perfect square.
* For option B: $12180$ ends in a single zero. A perfect square can never end in an odd number of zeroes.
* For option D: $100100$ is a six-digit number. Let's evaluate its square root: $\sqrt{100100}\approx 316.38$, which is not an integer. Since $100100$ is a six-digit number that is clearly not a perfect square, it satisfies the criteria of the question perfectly.