An obtuse angle is an angle that measures more than 90° but not eauql or greater than 180°.
COA is the sum of COB and BOA, which is 90°, so COA is not obtuse.
BOA is 60°, so it is not obtuse.
DOA is a straight line, so it is 180°, so not obtuse.
DOB is COD plus COB. Since COA is 90°, DOC is also 90°, so DOC ples COB is 120°, which is between 90° and 180°, so DOB is obtuse and the correct alternative is C.