In hypothesis testing, a researcher always starts by assuming the position that is easiest to test statistically, that there is genuinely no effect or no difference between the groups being compared, this default starting assumption is what statisticians call the null hypothesis. The study is then designed to see whether the collected data provide strong enough evidence to reject this "no difference" assumption in favour of the alternative hypothesis, which claims a real difference or association does exist. The other two terms, "hypothesis of association" and "hypothesis of differences," both describe claims that some relationship or difference is present, the opposite of the "no difference" idea the question is asking about.
Therefore, the correct answer is Null hypothesis.