Smart ads for cars may riff off the seasonal weather or time of day.
The author uses the phrase "riff off" to mean:
А
begin with
B
copy
С
broadcast
D
adapt to

B. Copy

Explanation:

To riff off means to copy or borrow something and then use that to create something else.

When smart ads are said to riff off the seasonal weather or time of the day, they mean that the ad copies that weather and puts it into the ad to show how the car performs in such weather.

For instance if it is winter and snowing alot, the ad would show how the car runs in snow to convince people that the car is good in the current weather.