Decarburization is not a casting defect.

1. Hot tear :A crack that develops in a casting due to high residual stress.

2. Blow hole :It is fairly large, well rounded cavity produced by gases which displace the molten metal at the cope surface of a casting.

3. Scab :Due to hydrostatic pressure of liquid metal in the riser, if the impressionappears in the roof of casted part, it is called scab.

4. Decarburization :It may be described as a metallurgical process in which the surfaces of steel is depleted of carbon by heating above the lower critical temperature or by chemical action.

Hence, the correct option is (D).