Which of these four values is the smallest?

Turn each into a decimal: $3^{-2}=\frac{1}{9}\approx 0.111,2^{-4}=\frac{1}{16}=0.0625,5^{-1}=0.2,10^{-1}=0.1$. The smallest is $2^{-4}$. Picking $10^{-1}$ because 10 is the biggest base is the trap — with a negative exponent the biggest denominator wins, and $16>10$.