**Core Concept**
Amniocentesis is a prenatal diagnostic test that involves withdrawing a sample of amniotic fluid from the uterus to detect chromosomal abnormalities, such as Down's syndrome, in the fetus. This test is typically performed after 15 weeks of gestation.

**Why the Correct Answer is Right**
Amniocentesis is the investigation of choice in a pregnant lady at 18 weeks of pregnancy with a past history of delivering a baby with Down's syndrome. This is because amniocentesis can provide a definitive diagnosis of chromosomal abnormalities, such as Down's syndrome, through the analysis of fetal cells in the amniotic fluid. The test has a high accuracy rate and can also detect other genetic disorders. Amniocentesis involves the insertion of a fine needle into the amniotic sac to collect a sample of amniotic fluid, which is then analyzed for the presence of abnormal fetal cells.

**Why Each Wrong Option is Incorrect**
**Option A:** The triple screen test is a screening test that measures the levels of certain substances in the maternal blood to detect the risk of chromosomal abnormalities, such as Down's syndrome. However, it is not a diagnostic test and has a lower accuracy rate compared to amniocentesis.

**Option C:** Chorionic villous biopsy, also known as chorionic villus sampling (CVS), is a prenatal diagnostic test that involves removing a sample of chorionic villi from the placenta to detect chromosomal abnormalities. However, CVS is typically performed earlier in pregnancy, between 10-13 weeks, and carries a higher risk of miscarriage compared to amniocentesis.

**Option D:** Ultrasonography is a prenatal imaging test that uses sound waves to create images of the fetus and placenta. While ultrasonography can detect anatomical abnormalities and estimate the risk of chromosomal abnormalities, it is not a diagnostic test and cannot provide a definitive diagnosis of Down's syndrome.

**Clinical Pearl / High-Yield Fact**
It is essential to note that amniocentesis carries a small risk of miscarriage, and the procedure should only be performed after careful counseling and consideration of the risks and benefits. Additionally, amniocentesis should only be performed in a hospital setting with experienced healthcare providers.

**✓ Correct Answer: B. Amniocentesis**