Correct answer: 40 m
The shortest stopping distance is determined by the maximum deceleration due to static friction. The deceleration is given by \( a = \mu g \), where \( \mu = 0.5 \) and \( g = 9.8 \, \text{m/s}^2 \). Converting the speed to \( 20 \, \text{m/s} \), and using \( v^2 = u^2 - 2as \), we find \( s = 40 \, \text{m} \).