**Core Concept**
Spontaneous delivery of placenta refers to the natural separation and expulsion of the placenta from the uterus after childbirth. This process is often compared to manual extraction of placenta, where the placenta is manually removed by a healthcare provider. The key principle being tested here is the association between spontaneous placental delivery and the risk of complications.

**Why the Correct Answer is Right**
The correct answer is related to the reduction of postpartum hemorrhage (PPH) risk. Spontaneous placental delivery allows for the uterus to contract and expel the placenta, which helps to reduce the risk of PPH. This is because the uterus can contract more effectively, reducing the amount of blood that escapes from the placental bed. In contrast, manual extraction of placenta can sometimes lead to PPH due to the manipulation of the uterus and the placental bed.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because spontaneous placental delivery does not necessarily reduce the risk of infection. In fact, manual extraction of placenta may be associated with a slightly higher risk of infection if not performed correctly.

**Option B:** This option is incorrect because spontaneous placental delivery does not reduce the risk of uterine atony. Uterine atony is a condition where the uterus fails to contract after childbirth, leading to PPH. Spontaneous placental delivery does not guarantee uterine contraction.

**Option C:** This option is incorrect because spontaneous placental delivery does not reduce the risk of retained placental tissue. Retained placental tissue is a condition where part of the placenta remains inside the uterus after delivery. This can occur regardless of whether the placenta is delivered spontaneously or manually.

**Option D:** This option is incorrect because spontaneous placental delivery does not reduce the risk of endometritis. Endometritis is a condition where the lining of the uterus becomes inflamed, often due to infection. Spontaneous placental delivery does not guarantee a reduction in the risk of endometritis.

**Clinical Pearl / High-Yield Fact**
A mnemonic to remember the complications associated with manual extraction of placenta is "PPH (Postpartum Hemorrhage), UTI (Urinary Tract Infection), and Retained placenta (RP)."

**Correct Answer:** B.