**Core Concept**
Human chorionic gonadotropin (hCG) is a hormone produced by the placenta during pregnancy. It is a key indicator of pregnancy and is typically detectable in the blood and urine around 6-8 weeks of gestation.

**Why the Correct Answer is Right**
The best confirmation for pregnancy at six weeks is the detection of beta-hCG levels in the blood. This is because hCG levels increase exponentially during early pregnancy, making it a sensitive indicator of pregnancy. Blood tests for beta-hCG can detect as little as 1-2 mIU/mL, making it a reliable method for confirming pregnancy at this stage. The sensitivity of blood tests for beta-hCG is higher than urine tests, making it a more accurate method for confirming pregnancy.

**Why Each Wrong Option is Incorrect**
* **Option A:** Ultrasound is not the best method for confirming pregnancy at six weeks because it may not be able to visualize the embryo or placenta in detail at this stage.
* **Option B:** Serum progesterone levels may be elevated during early pregnancy, but they are not specific for pregnancy and can be elevated in other conditions as well.
* **Option D:** Urine pregnancy tests are less sensitive than blood tests for beta-hCG and may not detect pregnancy at six weeks.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the sensitivity and specificity of pregnancy tests can vary depending on the type of test and the individual's hormone levels. A negative result does not necessarily rule out pregnancy, and repeat testing may be necessary to confirm or rule out pregnancy.

**Correct Answer: C. Blood test for beta-hCG.**