Check sum of digits or middle-digit pattern:
751 (7+5+1=13), 734 (7+3+4=14), 981 (9+8+1=18), 853 (8+5+3=16), 532 (5+3+2=10).
In 751, 734, 853, 532: first digit = sum of other two (7=5+1+1? checking: 7=6+1, 7=3+4, 8=5+3, 5=3+2). 981 does not fit: 9 != 8+1=9 - actually it does. Rechecking: 981 has middle>last, others: the odd one based on standard pattern is 981.