A null hypothesis is

The correct answer is A. When there is no difference between the variables.

A null hypothesis is a statement that there is no difference between the variables being studied. It is often used in research to test the effectiveness of a new treatment or intervention. The null hypothesis is typically denoted by $H_0$.

The alternative hypothesis is the statement that there is a difference between the variables. It is often denoted by $H_1$.

The null hypothesis is tested by collecting data and calculating a p-value. The p-value is the probability of obtaining the results that were observed if the null hypothesis were true. If the p-value is less than or equal to a pre-specified significance level, then the null hypothesis is rejected. This means that there is evidence to suggest that there is a difference between the variables.

Here is a brief explanation of each option:

• Option A: When there is no difference between the variables. This is the correct answer. The null hypothesis states that there is no difference between the variables being studied.
• Option B: The same as research hypothesis. This is incorrect. The null hypothesis is not the same as the research hypothesis. The research hypothesis is the statement that there is a difference between the variables.
• Option C: Subjective in nature. This is incorrect. The null hypothesis is not subjective in nature. It is a statement that can be tested using data.
• Option D: When there is difference between the variables. This is incorrect. The null hypothesis states that there is no difference between the variables.