Who do Seventh Day Adventist think the Harlot is in Revelation chaps 17 & 19? According to Seventh Day Adventist’s Who/what is the harlot noted in Revelation?
Why do you or your source think that answer is correct?
From a Seventh-Day Adventist perspective, and also what many of the reformers believed, the Bible points to the harlot of Revelation as the Roman Catholic system.


A woman in prophecy and in Jesus' parables (10 virgins) represents a church or group of believers. (Revelation 12:1-6, 10 virgins, etc.)
A harlot represents a church that has been unfaithful to the teachings of God.



  Cause Jerusalem to know her abominations. ... You trusted in your own
  beauty, played the harlot” (Ezekiel 16:2, 15).



She persecuted the saints (Rev 17:6).


4.She was dressed in purple and scarlet (Rev 17:4). The pope often wears the royal color 
of purple at important functions, and red is the color of the robes of Catholic cardinals.

5.The seven heads of the beast (Rev 17:3) upon which the woman is seated are seven mountains (Rev 17:9). It is well-known that Rome, the headquarters of the papacy, is built upon seven hills, or mountains.

6.The beast is guilty of blasphemy (Rev 17:3), a point that also clearly fits the papacy, if you look at the Biblical definition of blasphemy (claims to forgive sins and claims to be God)

7.She ruled “over the kings of the earth” (Rev 17:18). Alexander Flick says that by the 13th century, the pope was “at least in theory ... the ruler of the whole world in temporal and spiritual affairs.” This point could fit no other earthly kingdom or government. The papacy is described in Revelation 17 too clearly for doubt.

This will likely ruffle some feathers and get down-voted around here, but some people still do believe this, among them are SDAs. Also, pretty much all the reformers of old believed this as well. Huss, Wycliffe, Luther, Calvin, Zwingli, Melanchthon, Cranmer, Tyndale, Latimer, Ridley, and many others.

For a in-depth, Biblical study, see The Other Woman.