"From that single plant, which has basically no variation, you create the whole species, and then the variation is only the novel mutations that have occurred since that event" -> Low DNA variation
"there is substantial variation in physical characteristics... We are always talking about the low variability at the DNA level, but there is great variability at the structural level, at the chromosomal level, at the level of deletions ... insertions" -> All other levels have high variation
-> Option 1: Structural level - Has "great variability"
-> Option 2: Chromosomal level - Has "great variability"
-> Option 3: Deletions and insertions - Has "great variability"
-> Option 4: DNA level - Has "low variability" ✓
The answer is Option 4 - Variability at the DNA level.