Translation:What is written in the contract?
This should be "in" the contract though it's probably OK to also accept "on".
Weird that the suggested one uses “on” on this page… but “on” is not accepted.
"What is in the contract" means the same as "what is written in the contract." The "written" is obvious enough that you can drop it and the sentence does not lose any meaning. Without "written" should also be an acceptable answer.
To me they have very different meanings. "In" suggests the contract as printed out, "on" suggests a handwritten alteration before signing the contract.