Answer: Type I error
Explanation:
A type 1 error is also referred to as the false positive and it is when a true null hypothesis is incorrectly rejected by the researcher.
On the other hand, type II error which is also refered to as the false-negative is when a null hypothesis which is false is failed to be rejected by the researcher but rather accepted.
Based on the information given in the question, since the data leads to the rejection of the null hypothesis such was later found out to be true, then a type I error has occured.