You are designing a 12-week randomized controlled trial to test whether a psyllium supplement reduces LDL-cholesterol compared with placebo. Which study design element combination best synthesizes physiological understanding and methodological rigor?

Double-blind, placebo-controlled randomization; daily psyllium dose known to form viscous gel; standardized background diet low in other viscous soluble fibers; primary outcome LDL measured fasting at baseline and 12 weeks; control for weight and statin use in analysis.

Single-arm observational study giving psyllium to all participants for 12 months with LDL measured at the end and historical controls used for comparison, with no randomization or blinding.

Crossover study with one day of psyllium and one day of placebo separated by one week, measuring immediate postprandial LDL and HDL to capture rapid cholesterol-lowering effects of viscous fibers.

Open-label trial where participants choose psyllium or placebo; no dietary standardization; primary outcome change in self-reported satiety; no baseline lipid measurement required.