A 'de novo' trial means a completely new trial of a matter, conducted as if the original trial had not taken place. The Supreme Court has repeatedly held that this is an extreme measure.
(a) is incorrect. Minor procedural omissions that do not cause a failure of justice are curable under S.465 CrPC and are not grounds for a new trial.
(b) is correct. Courts have held that a de novo trial wastes judicial time and resources and should be ordered only in exceptional circumstances where there has been a fundamental defect in the original trial, leading to a gross miscarriage of justice. It is a last resort.
(c) is incorrect. A de novo trial is typically ordered by an appellate or revisional court, not the original trial court itself.