Correct spelling


Correct spelling, explanation: this form, stopping, with consonant p doubled, is the correct one, because the word stop ends with a combination of letters: consonant-vowel-consonant. According to the rule, such combination means that the last consonant is doubled when we add any suffix to a word. Therefore stopping is created as a correct form, and stoping is a mistake*.

Definition of stopping:
verb, -ing form of stop; to cease to go or do something, not moving any more
We were stopping at every clothes store to find the right shoes.
This bus isn’t stopping at the square today because there is a fair there.

Incorrect spelling


Incorrect spelling, explanation: this form is wrong since p should be doubled (as the last consonant of a consonant-vowel-consonant combination that ends the word). Therefore when the suffix -ing is added to the word stop, the result is stopping, not stoping.
*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 suffix.