In chain surveying, perpendiculars to the chain line, are set out by

ANS:D - an optical square

In chain surveying, perpendiculars to the chain line are typically set out using a prismatic compass. A prismatic compass is a type of compass that allows surveyors to measure horizontal angles accurately. It can be used to establish perpendiculars to the chain line by sighting on a distant point and then marking the perpendicular line at a right angle to the chain line. This helps in accurately laying out the survey area and establishing the boundaries or features of interest.