What is a type I error?

When the researcher rejects the null hypothesis, but the null hypothesis is actually true (and there really are no differences).

When the researcher rejects the null hypothesis, and the null hypothesis is false (and there really are differences).

When the researcher accepts the null hypothesis, and the null hypothesis is false (and there really are differences).

When the researcher accepts the null hypothesis, but the null hypothesis is actually true (and there really are no differences).