Correct answer: [ML⁻¹T⁻¹]
The given formula F = 6πηav relates force (F) to velocity (v) and radius (a). Rearranging for η, we get η = F / (6πav). Since F has dimensions [MLT⁻²], a has dimensions [L], and v has dimensions [LT⁻¹], the dimensions of η are [MLT⁻²] / ([L][LT⁻¹]) = [ML⁻¹T⁻¹].