To make a educational guess based on the your observations
"Frequency" just means "often-ness" ... how often something happens.
It's always expressed as
<em>(number of happenings) / (some period of time) .</em>
I believe it is D. Earth spinning on it's axis.
Explanation:
Given that,
Two resistors of resistance 6 ohm and 3 ohm are connected in series and then in parallel.
For series combination,

For parallel combination,

When 6 ohm and 3 ohm are in series,

When 6 ohm and 3 ohm are in paralle,

So, the equivalent resistance in series combination is 9 ohms and in parallel combination it is 2 ohms.
gawaingnpang nkabuhayan, hamon at oportinidad