To find the combined water capacity of the two water towers, we need to add the capacities in scientific notation.
The two capacities are:
- Tower 1: \( 7.35 \times 10^5 \) gallons
- Tower 2: \( 9.78 \times 10^5 \) gallons
When adding numbers in scientific notation, you must ensure both numbers have the same exponent. In this case, both numbers have the exponent \( 10^5 \), so we can directly add the coefficients:
\[ 7.35 + 9.78 = 17.13 \]
Now, we can express this sum in scientific notation. Since \( 17.13 \) is greater than \( 10 \), we need to convert it to proper scientific notation:
\[ 17.13 = 1.713 \times 10^1 \]
Now, we combine it with \( 10^5 \):
\[ 1.713 \times 10^1 \times 10^5 = 1.713 \times 10^{1+5} = 1.713 \times 10^6 \]
Thus, the combined water capacity in scientific notation is:
\[ \boxed{1.713 \times 10^6} \]
This matches option 1 in your list.