To find out how many times less \( 10^a \) is than \( 10^6 \), we need to divide \( 10^6 \) by \( 10^a \):
\[ \frac{10^6}{10^a} = 10^{6-a} \]
This expression tells us how many times \( 10^a \) is less than \( 10^6 \).
For example, if \( a = 2 \), then \( 10^{6-2} = 10^4 \), meaning \( 10^2 \) is \( 10^4 \) times less than \( 10^6 \).
Hence, if we want to express how many times less \( 10^a \) is than \( 10^6 \), we denote it as:
\[ 10^{6-a} \]
To express the answer as an integer power of 10, we need to calculate \( 6 - a \).
Thus, \( 10^a \) is \( 10^{6-a} \) times less than \( 10^6 \).
From the answer choices given:
- \( 10^4 \)
- \( 10^{-4} \)
- \( 0.0001 \)
- \( 10,000 \)
Since \( 10000 = 10^4\), it is clear that the correct expression for how many times less \( 10^a \) is than \( 10^6 \) (for some integer \( a \)) can be \( 10^4 \).
Therefore, the correct answer would be:
\( 10^4 \).