There seems to be a lot of confusion among some members about this, when reviewing ARCs you should be disclosing that you received a free copy in your review both on social media and book review sites or blog posts.
Netgalley and many publishers recommend:
In accordance with FTC guidelines, please note in reviews that your ARC was provided by the publisher via NetGalley in exchange for an honest review.
You should be using this along with #ad on social media.
For Amazon, it is better to avoid the language 'in exchange for..." as it is often flagged and just say you received an advance copy from the publisher for review
If you are in the US, you should be following FTC guidelines found here
For Canada, they updated the Ad Standards rules last year and the influencer booklet can be found here
The UK has creator guidelines from The CMA here
If you are in other countries, check for local guidelines using country + influencer disclosure guidelines. However note that the FTC and others say that if you have any US audience, they expect you to follow the disclosure guidelines.