The correct answer is A. 10° to 30°.
Lacing bars are used in reinforced concrete columns to provide lateral stability and to prevent buckling. The angle of inclination of the lacing bar with the longitudinal axis of the column should preferably be between 10° and 30°. This is because at this angle, the lacing bar provides the most effective lateral support to the column. If the angle is too small, the lacing bar will not be effective in preventing buckling. If the angle is too large, the lacing bar will be more likely to buckle itself.
Option B is incorrect because the angle of inclination should not be greater than 30°. Option C is incorrect because the angle of inclination should not be less than 10°. Option D is incorrect because the angle of inclination should not be 90°.