Post term labour is seen in:: (A) Hydramnios, (B) PID, (C) Anencephaly, (D) Multiple pregnancy
Answer is C. Causes of Post-term pregnancy:

Wrong dates: due to inaccurate LMP (most common).
Biologic variability (Hereditary) may be seen in the family.
Maternal factors: Primipara/elderly multipara/H/o previous prolonged pregnancy, sedentary habit.
Fetal factors: Congenital anomalies: Anencephaly – (Abnormal fetal HPA axis), adrenal hypoplasia (Diminished fetal cortisol response).
Placental factors: Sulphatase deficiency (Low oestrogen).