$\sigma_{A \leq 100}(r)$
$r$ has $1000$ tuples
Values for A among the tuples are uniformly distributed in the interval $[0, 500].$ This can be split to $5$ mutually exclusive (non-overlapping) and exhaustive (no other intervals) intervals of same width of $100$ $([0-100], [101-200], [201-300], [301-400], [401-500],$ $0$ makes the first interval larger - this must be a typo in question) and we can assume all of them have same number of values due to Uniform distribution. So, number of tuples with A value in first interval should be
$\frac{\text{Total no. of tuples}}{5} = 1000/5 = 200$
Correct Answer: $D$