Step 1: Audiometric charting uses a standard colour code so left and right thresholds never get confused.
Step 2: The right ear is always drawn in red and the left ear in blue; bone and air conduction are separated by symbol type rather than colour.
Step 3: Remember the mnemonic Red equals Right to fix the colour to the side.
Step 4: Hence the red line on the audiogram belongs to the right ear, eliminating bone conduction, air conduction and left ear as answers.
\[\boxed{\text{Right ear}}\]