**Core Concept**
A tubal pregnancy, also known as an ectopic pregnancy, occurs when a fertilized egg implants outside the uterus, typically in a fallopian tube. This condition is a medical emergency that requires prompt diagnosis and treatment.

**Why the Correct Answer is Right**
The most frequent predisposing factor for tubal pregnancy is a history of pelvic inflammatory disease (PID). PID is an infection of the female reproductive organs, which can cause inflammation and scarring in the fallopian tubes. This scarring can lead to a higher risk of ectopic pregnancy. The scarring can disrupt the normal transport of the fertilized egg through the fallopian tube, causing it to implant in the tube instead of the uterus.

**Why Each Wrong Option is Incorrect**
**Option A:** Previous cesarean section is not a significant risk factor for tubal pregnancy. While cesarean section can increase the risk of future ectopic pregnancies, it is not the most frequent predisposing factor.
**Option B:** History of infertility is not a direct predisposing factor for tubal pregnancy. However, women with a history of infertility may be more likely to have underlying conditions such as PID that increase the risk of ectopic pregnancy.
**Option C:** Smoking is a risk factor for ectopic pregnancy, but it is not the most frequent predisposing factor. Smoking can damage the fallopian tubes and increase the risk of ectopic pregnancy, but it is not as common a cause as PID.

**Clinical Pearl / High-Yield Fact**
Women with a history of PID should be counseled on the increased risk of ectopic pregnancy and advised to use effective contraception to prevent future pregnancies.

**Correct Answer:** D.