Correct Answer: 433

Solution : The highest common factor (HCF) of 6, 8, and 9 is 1
For 291:
Remainder when divided by 6: 291 mod  6 = 3
Remainder when divided by 8: 291 mod  8 = 3
Remainder when divided by 9: 291 mod  9 = 0
It doesn't satisfy the condition for the highest common factor.
For 575:
Remainder when divided by 6: 575 mod  6 = 5
Remainder when divided by 8: 575 mod  8 = 7
Remainder when divided by 9: 575 mod  9 = 2
It doesn't satisfy the condition for the highest common factor.
For 506:
Remainder when divided by 6: 506 mod  6 = 2
Remainder when divided by 8: 506 mod  8 = 2
Remainder when divided by 9: 506 mod  9 = 7
It doesn't satisfy the condition for the highest common factor.
For 433:
Remainder when divided by 6: 433 mod  6 = 1
Remainder when divided by 8: 433 mod  8 = 1
Remainder when divided by 9: 433 mod  9 = 1
It satisfies the condition for the highest common factor.
Hence, the correct answer is 433.