Explanation
Correct answer: C.
Choice C is the best answer because "that said" logically signals that the information that follows—that the crystals of microcrystalline minerals can be seen with a microscope—is an exception to the previous information about the crystalline structure of minerals not being visible under a microscope.
Why the other choices are wrong
Choice A
Choice A is incorrect because "thus" illogically signals that the information that follows is a direct result or consequence of the fact that the crystals of microcrystalline minerals aren’t visible to the naked eye. Instead, the fact that they can be seen under a microscope is an exception to the previous information about crystalline minerals.
Choice B
Choice B is incorrect because "for example" illogically signals that the information that follows exemplifies the fact that the crystals of microcrystalline minerals aren’t visible to the naked eye. Instead, the fact that they can be seen under a microscope is an exception to the previous information about crystalline minerals.
Choice D
Choice D is incorrect because "similarly" illogically signals that the information that follows is similar or comparable to the fact that the crystals of microcrystalline minerals aren’t visible to the naked eye. Instead, the fact that they can be seen under a microscope is an exception to the previous information about crystalline minerals.