We have to simplify: √ ( - 28 ) We know that √ (-1 ) = i ( the imaginary unit ) √ ( - 28 ) = √ 28 * ( - 1 ) = i √ ( 28 ) = i √ ( 4 * 7 ) = 2 i √( 7 ) But there is not such solution among the options. Only correct answer ( using the imaginary unit ) is: C. i (square root) 28