The disease characterised by extreme muscular weakness and brownish pigmentation of buccal

cavity and skin is

The correct option is B Addison's disease
Addison's disease is a disorder that occurs when insufficient amounts of hormones (too little cortisol and insufficient aldosterone) produced by adrenal glands. This results in a diffuse dark pigmentation of the skin and the oral mucosa. There are extreme muscle weakness and fatigue.
Thus, the correct answer is the option B.