A clinical psychologist wants to compare the effectiveness of Cognitive Behavioural Therapy (CBT) versus Mindfulness-Based Cognitive Therapy (MBCT) for reducing anxiety. She randomly assigns 30 participants to either the CBT group ($n_1=15$) or the MBCT group ($n_2=15$). After 8 weeks, anxiety reduction is measured using a standardised clinical scale that yields interval-level data. A Shapiro-Wilk test indicates the scores in both groups are normally distributed, and Levene's test confirms homogeneity of variance. Which of the following is the most appropriate statistical test to analyse this data?