_______________ have to label their ______________ posts in order to bring about transparency and ________________ the interests of the customers.

For the first blank, we need a noun referring to people who post content online and may be paid to promote products — typically social media influencers (based on the use of the word post).  For the second blank, we need a word that describes the type of posts they are expected to label, which can be inferred to be promotional content. For the third blank, we want a verb that indicates protecting or ensuring customers' well-being, in line with the idea of transparency.

Influencers is the best choice for the first blank. Promotional is the correct fit for the second blank, describing content meant to advertise. Safeguard is ideal for the third blank, as it conveys the idea of protecting customer interests.

Hence, option A is the best choice.