Step 1: Match each letter of BEAUTIFUL to its digit.
BEAUTIFUL has 9 letters, B, E, A, U, T, I, F, U, L, and the code 573041208 has 9 digits, 5, 7, 3, 0, 4, 1, 2, 0, 8. Lining them up gives B=5, E=7, A=3, U=0, T=4, I=1, F=2, U=0, L=8.
Step 2: Check the mapping against BUTTER.
BUTTER has letters B, U, T, T, E, R, and using the digits found so far, B=5, U=0, T=4, T=4, E=7, and the code 504479 confirms these, so R must be 9 to complete the last digit.
Step 3: Apply the confirmed letter-digit key to FUTURE.
FUTURE has letters F, U, T, U, R, E. Using F=2, U=0, T=4, U=0, R=9, E=7, the digits in order are 2, 0, 4, 0, 9, 7.
Step 4: Rule out the other options.
Option A (201497) and option D (204079) place the digits for T, U and R in the wrong slots, and option C (704092) starts with the digit for E instead of F, so none of these match the letter order of FUTURE.
Final Answer:
Reading off the key in the order F-U-T-U-R-E gives 204097. \[ \boxed{204097} \]