27) E

When the material is living the ratio is 1000 : 1015

When the material died, the ration became 100 : 1015

This means that the ratio decreased by a factor of 10.

1 half-life = 6000 years

Let number of half-lives be M

1000/100 = 2M

10 = 2M

We know 23 = 8 and 24 = 16

So number of half-lives is a decimal between 3 and 4. (Let us take it as 3.5)

Number of half-lives = 3.5
Number of years = 6000 x 3.5 = 21,000 years
This is close to 20,000 so E is the answer.