Spot the term that breaks the pattern: 1, 3, 7, 10, 15, 21, 28.

Explanation

1. 01

   The last three differences are 5, 6, 7 (10 to 15, 15 to 21, 21 to 28): they rise by one each time.

2. 02

   The full pattern is +2, +3, +4, +5, +6, +7, so the third term is $3 + 3 = 6$, not 7.

3. 03

   With 6 in place: 1, 3, 6, 10, 15, 21, 28.