How do you spell “agree”?
The word "according to" comes from the adjective "consonant", in which the prefix "with" is written. Therefore, the word "according to" also begins with "with". In addition, the word "according to" ends with the letter "o". Moreover, if you replace the ending with “a” or “en”, these will already be other words. So, the correct option: according to.
It is also worth noting that if the word "according to" is used as a preposition, then it requires a word after itself in the dative case. For example, it is wrong to speak and write “according to a plan”, correctly “according to a plan”.