The largest six digit telephone number that can be formed by using digits 5, 3, 4, 7, 0, 8 only once is 875403.

False.

Arranging the numbers in descending order, we get :
8 > 7 > 5 > 4 > 3 > 0.
Thus the largest six digit number that can be formed using these digits is 875430.
875430 ≠ 875403