Rounding is simplifying a number to a certain place value. To round the decimal drop the extra decimal places, and if the first dropped digit is 5 or greater, Rounding up the last digit that you keep. If the first dropped digit is 4 or smaller, Rounding down (keep the same) the last digit that you keep.
a = s.tu,
b = m.n
for simplicity let us also assume s = m
(1) When a is rounded to the nearest integer, the result is less than a
This implies t is less than 4 as per the rule. It can be anything < 4 hence Insufficient
(2) When b is rounded to the nearest integer, the result is greater than b.
This implies n >= 5 as per the rule. still we cannot find t.hence Insufficient
if we combine above two statements t must be 4 so n must be at the minimum 5 when a is rounded to the nearest tenth. Answer C