Which of these did the Jerusalem council's letter tell Gentile believers to abstain from?

Answer: Blood

The letter asked them to abstain from food sacrificed to idols, from blood, from strangled animals and from sexual immorality.

“It seemed good to the Holy Spirit and to us not to burden you with anything beyond these essential requirements: You must abstain from food sacrificed to idols, from blood, from the meat of strangled animals, and from sexual immorality. You will do well to avoid these things. Farewell.”— Acts 15:28-29 (BSB)