Viewable cost–per–thousand impressions (vCPM):

• Counts An Ad As “Viewable” When 75% Of It Is Visible On The Screen For 5 Seconds Or More
• Lets You Pay Only For Impressions That Become Viewable
• Applies To All Ads That Appear In Google Search And On The Display Network
• Lets You Bid Based On Adwords’ Projected Views Of Your Ad

The correct answer is:

• Lets You Pay Only For Impressions That Become Viewable.