Human female sex chromosomes.

**The reasoning** In humans, biological sex is determined by a pair of chromosomes called sex chromosomes. Females have **two X chromosomes (XX)**, while males have one X and one Y chromosome (XY). This is basic genetics — you inherit one sex chromosome from each parent. A mother (XX) can only give an X, while a father (XY) can give either X or Y. If you get X from both parents, you develop as female (XX). **Why the wrong options tempt you** - **A) XY** — This is the *male* combination. Easy to mix up if you're rushing or didn't memorize which is which. - **C) YY** — Biologically impossible in humans! You *must* get at least one X chromosome to survive. YY can't exist naturally. - **D) XO** — This is actually a chromosomal disorder called Turner syndrome (only one X, no second sex chromosome). It's not the normal female pattern. **Quick takeaway** Remember: **"XX = eXtra X = female"** — two X's make a girl, just like you need two to make a pair!