A null hypothesis is

A null hypothesis (H0) is the default, testable statistical statement used as the starting point of a hypothesis test: it asserts that there is no difference, no effect, and no relationship among the variables being studied, and the researcher's task is to determine whether the sample evidence is strong enough to reject it.

In this item, only one option matches that definition exactly: the option stating that there is no difference between the variables being compared is the standard definition of a null hypothesis used across research-methodology and statistics texts.

• the same as research hypothesis — the research (alternative) hypothesis is the researcher's claim that an effect or relationship does exist; it is the complementary counterpart to a null hypothesis, not another name for it.

• subjective in nature — a null hypothesis is stated as a precise, testable proposition (for example, "there is no difference between the means"), not as a matter of personal opinion or judgement.

• when there is difference between the variables — this describes the alternative hypothesis, which is the logical negation of the null hypothesis, not the null hypothesis itself.

Cross-checking against the alternative hypothesis (defined as the exact negation of the null hypothesis) confirms the pairing: "no difference" corresponds to the null hypothesis and "difference" corresponds to the alternative hypothesis. So the correct answer is: when there is no difference between the variables.