$19)$ Which of the following investments provides the least variability and risk?

Investment A: Avg. return $=12\%,$ std$.$ deviation $=18\%$

Investment B: Avg. return $=27\%,$ std$.$ deviation $=5\%$

Investment C: Avg. return $=15\%,$ std$.$ deviation $=2\%$

Investment D: Avg. return $=7\%,$ std$.$ deviation $=8\%$