The correct answer is:  [C]:  " 8.64 mm² " .
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Note:  
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Area of a triangle = (½)*(base length)*(height; that is, "perpendicular height");
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or:  A = (½) * b * h ;
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Given:  "b = 4.8 mm" ; 
            "h = (¾) * b = (¾)* (4.8 mm) = 3.6 mm ;
               → h = 3.6 mm ;
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  →  A = (½) * b * h  ;
           = (½) * (4.8 mm) * (3.6 mm) ;
    →  A  =  8.64 mm² ; which is:  Answer choice: [C] . 
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