To answer this question, we can use the concept of similar triangles.
Let's consider the first situation where the pillar is 8 feet tall and casts a shadow that is 4 feet long.
We can set up a proportion between the height of the pillar and the length of its shadow:
$\frac{8}{4} = \frac{16}{x}$
Simplifying this proportion, we have:
$\frac{2}{1} = \frac{16}{x}$
Cross-multiplying, we get:
$2x = 16$
Dividing both sides by 2, we find:
$x = 8$
Therefore, when the pillar is 16 feet tall, the length of its shadow would be 8 feet.
So, the correct answer is A) 8.