**Core Concept**
Human chorionic gonadotropin (hCG) is a hormone produced during pregnancy that is detectable in maternal serum and urine. The sensitivity of hCG tests has improved significantly, allowing for earlier detection of pregnancy.

**Why the Correct Answer is Right**
The correct answer is 8 to 9 days after ovulation because hCG is produced by the developing embryo after implantation in the uterus. This typically occurs 6-8 days after fertilization. The hCG produced by the embryo stimulates the corpus luteum to continue producing progesterone, which is essential for maintaining pregnancy. The hCG level increases rapidly after implantation, making it detectable in maternal serum or urine by 8-9 days after ovulation. This is why sensitive hCG tests can detect pregnancy at an early stage.

**Why Each Wrong Option is Incorrect**
**Option A:** 3 to 4 days is too early for hCG to be detectable in maternal serum or urine, even with sensitive tests. This is because hCG levels are still rising after implantation and have not yet reached detectable levels.

**Option C:** 12 to 14 days is too late for hCG to be detectable in maternal serum or urine. By this time, hCG levels have already peaked and are beginning to decline.

**Option D:** 20 to 21 days is much too late for hCG to be detectable in maternal serum or urine. By this time, the pregnancy test would likely be positive, but the hCG levels would be decreasing, making it less sensitive for detection.

**Clinical Pearl / High-Yield Fact**
It's essential to note that hCG levels can vary significantly between individuals and pregnancies. A sensitive hCG test can detect pregnancy earlier than an insensitive test, but it's still possible for some pregnancies to be missed due to low hCG levels or other factors.

**✓ Correct Answer: B. 8 to 9 days**