**Core Concept**
Crossing over is a genetic event that occurs during prophase I of meiosis, where homologous chromosomes exchange segments of DNA. This process increases genetic diversity and is essential for proper chromosome segregation.

**Why the Correct Answer is Right**
Crossing over occurs between **non-sister chromatids** of **homologous chromosomes**, specifically during the **diplotene stage** of prophase I. The exchange of genetic material happens via the formation of chiasmata, where DNA breaks are repaired through recombination between non-sister chromatids. This ensures proper alignment and separation of homologous chromosomes in meiosis I.

**Why Each Wrong Option is Incorrect**
Option A: Although crossing over occurs during diplotene, the statement is incomplete and misleading. The key point is not just the stage but the **type of chromatids involved**, making this option less accurate than B.
Option C: Sister chromatids are identical and do not undergo crossing over; they are separated in anaphase II. This would not generate genetic diversity.
Option D: Non-homologous chromosomes do not pair or exchange material during meiosis; crossing over only occurs between homologous chromosomes.

**Clinical Pearl / High-Yield Fact**
Remember: **Crossing over = non-sister chromatids of homologous chromosomes**. This is a classic exam trap — students often confuse it with sister chromatid exchange, which does not occur in meiosis.

✓ Correct Answer: B. Between non sister chromatids of homologous chromosomes