The indwelling of the Holy Spirit is necessary for salvation and entering the kingdom of God. The Holy Spirit
sanctifies by attesting to the reality of Believer's faith, brands the Believer as a property of God's, and
guarantees his inheritance in the future completion of salvation
(2 Cor 1:21-22; 5:5).
The correct answer is "b" - the Holy Spirit sanctifies by assuring the Believer of the indwelling presence of
Jesus Christ (1 John 3:24).