Population is getting doubled every 12 years, and population in the year 2021 is 50 animals. So, after 12 years it will get doubled to 100 animals.

Now, let's check the options, where n stands for the number of years.

Option (d):

$P = 50 \times (2)^{n/12} = 50 \times 2^{12/12} = 50 \times 2 = 100$ animals.

For all other options, this value comes out to be way above 100.

Hence, option (D) is the correct answer.