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A
As deep as the saw allows
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B
About 1 inch below the material thickness
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C
Just deep enough to cut through the material — blade depth set approximately 1/4 inch deeper than the material thickness provides the safest and most efficient cut
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D
Half the material thickness
Why this is the answer
Setting the circular saw blade to just barely clear the bottom of the material (about 1/4 inch below the workpiece bottom) minimizes the amount of exposed blade, reducing kick-back risk and producing a cleaner cut. A deeper blade setting exposes more blade, increases the risk of kick-back if the blade binds, and cuts more aggressively. The correct setting: bottom of the blade clears the material with minimal overhang.
Source: ASVAB SI, Circular Saw Safety