Correct spelling


Correct spelling, explanation: according to the rule, that says if the word ends with a consonant-vowel-consonant combination, the last consonant is doubled with added suffix, stopping is spelled with double p. It is simply the word stop with suffix -ing – a suffix is added so the last consonant doubles. Therefore stoping is incorrect and stopping is the only correct form*.

Definition of stopping:
verb, -ing form of stop; to end moving or doing something
We should be stopping every 50 miles if we want to get there on time.
If you are an inexperienced hitch-hiker, you should know that stopping a car can last sometimes an hour or more.

Incorrect spelling


Incorrect spelling, explanation: in this word p should be doubled, since the word ends with the consonant-vowel-consonant combination (in such case when we add a suffix, we double the last consonant). Thus stoping is incorrect and the correct form is stopping, with double p.
*Notice: there is a term in geology, which is called stoping – it is a process of magma moving upwards, also known as magmatic theory. In this case, however, it is a separate noun, not a verb with a suffix.