Concept:The noun "means" (a way of doing something) is followed by "of", usually with a gerund (-ing form): "a means of + verb-ing".Solution:The blank is followed by "making", an -ing form.The standard expression is "as a means of making an extra income": baking bread is her way of earning extra money."A means for", "a means in" and "a means to making" are not the correct pattern here ("a means to an end" is a fixed phrase with a noun, not with an -ing verb).Correct sentence: "She bakes bread as a means of making an extra income."Answer:Option D (of)