What command will search for physical defects on a hard drive and will mark these defects so that
they are no longer used? (Please specify the command with no path information)

Answer: badblocks

Explanation:
From the man pages:
badblocks – search a device for bad blocks
badblocks [ -svwnf ] [ -b block-size ] [ -c blocks_at_once ] [ -i input_file ] [ -o output_file ] [ -p
num_passes ] [ -t test_pattern ] device [ last-block ] [ start-block ]