• Flow Cytometry in Cell Cycle Analysis:
• Flow cytometry is the benchmark analytical technique for determining cell cycle distribution by quantifying the cellular DNA content of thousands of individual cells in suspension:
• Cells are permeabilized and stained with a fluorescent DNA-binding dye (propidium iodide, DAPI).
• Fluorescence emission is strictly proportional to DNA content:
• $\text{G}_0/\text{G}_1$ phase cells contain diploid ($2n$) DNA.
• $\text{G}_2/\text{M}$ phase cells contain tetraploid ($4n$) DNA and display double the fluorescence intensity of $\text{G}_1$ cells.
• $\text{S}$ phase cells display an intermediate fluorescence intensity ($2n \lt \text{DNA} \lt 4n$).

Correct option: (4) Flow cytometry