\[1)\ 15i;\]
\[|z| = \sqrt{0^{2} + 15^{2}} = \sqrt{15^{2}} = 15.\]
\[2) - 21i;\]
\[|z| = \sqrt{0^{2} + ( - 21)^{2}} =\]
\[= \sqrt{21^{2}} = 21.\]
\[3) - 5 + 2i;\]
\[|z| = \sqrt{( - 5)^{2} + 2^{2}} =\]
\[= \sqrt{25 + 4} = \sqrt{29}.\]
\[4)\ \sqrt{3} - i;\]
\[|z| = \sqrt{\left( \sqrt{3} \right)^{2} + ( - 1)^{2}} =\]
\[= \sqrt{3 + 1} = 2.\]
\[5) - 1 - 4i;\]
\[|z| = \sqrt{( - 1)^{2} + ( - 4)^{2}} =\]
\[= \sqrt{1 + 16} = \sqrt{17}.\]
\[6)\ \sqrt{11} + \sqrt{5}i;\]
\[|z| = \sqrt{\left( \sqrt{11} \right)^{2} + \left( \sqrt{5} \right)^{2}} =\]
\[= \sqrt{11 + 5} = 4.\]