Final answer:
The sum of squares within groups measures the variability of the observed values around their respective means within each group.
Step-by-step explanation:
The correct answer to the question is b. error/within groups. The sum of squares within groups measures the variability of the observed values around their respective means within each group. This represents the variation within samples that is due to chance or individual factors. In the context of analysis of variance (ANOVA), the sum of squares between groups measures the variability among the different samples.