B. & C.
A is incorrect because the most complex level of organization is the organism itself, as a whole.
D is incorrect because a cell is what composes different tissues, it isn’t made of different tissues.
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B. The cell is the most basic structural unit of life. They are independent and can perform processes like excrete waste, take in nutrients etc.
C. There are so many different cell types such as muscle cells, stomach cells, bone, neurons, etc. meaning they “specialize” in one thing such as being a stomach cell.
Hope this helps!