**Answer:**

a. The average number of hashtags used in a social media post from a marketing agency is different than 7 hashtags.

**Step-by-step explanation:**

**A social media platform states that a social media post from a marketing agency has 7 hashtags, on average.**

This means that at the null hypothesis, we test if the mean is 7, that is:

**A digital marketing specialist studying social media advertising believes the average number of hashtags used in a post from a marketing agency is different than the number stated by the social media platform.**

Keyword is different, so at the null hypothesis, we test if the mean is different of 7, that is:

Thus, the correct answer is given by **option a.**