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To find the probability that a randomly selected day in July in Jacksonville, Florida does not have a thunderstorm, we first need to know the total number of days in July, which is 31 days.

If, on average, there are 9.9 days with afternoon thunderstorms, then the number of days without thunderstorms is:

\[
\text{Number of days without thunderstorms} = 31 - 9.9 = 21.1
\]

To find the probability of a day not having a thunderstorm, we calculate:

\[
\text{Probability (not having a thunderstorm)} = \frac{\text{Number of days without thunderstorms}}{\text{Total number of days}} = \frac{21.1}{31}
\]

Now, let's calculate this:

\[
\frac{21.1}{31} \approx 0.681
\]

To express this as a percentage:

\[
0.681 \times 100 \approx 68.1\%
\]

Therefore, the closest answer is:

**A. 68.06%**