To determine which exponential function represents the geometric sequence \( 40, 160, 640, \ldots \), we first need to analyze the pattern in the sequence.
1. **Identify the first term**: \( a_1 = 40 \)
2. **Identify the common ratio**:
- From \( 40 \) to \( 160 \), the ratio is \( \frac{160}{40} = 4 \).
- From \( 160 \) to \( 640 \), the ratio is \( \frac{640}{160} = 4 \).
- The common ratio is \( r = 4 \).
Next, we can express the \( n \)-th term of a geometric sequence with the formula:
\[
a_n = a_1 \cdot r^{(n-1)}
\]
Substituting the values from our geometric sequence:
- \( a_1 = 40 \)
- \( r = 4 \)
Thus, the formula for the \( n \)-th term becomes:
\[
a_n = 40 \cdot 4^{(n-1)}
\]
So, out of the given options, this corresponds to:
**\( a_n = 40(4)^{n-1} \)**
Therefore, the correct exponential function that shows the geometric sequence \( 40, 160, 640, \ldots \) is:
**\( a_n = 40(4)^{n-1} \)**.