1) Minimum number of people who attended the party: 24 (including the host)

2) 24 people would have received 24x25/2 = 300 chocolates. However, you asked for the maximum number of chocolates - it's possible that 25 or 26 people attended. If 26 attended then they would have received 26x27/2 = 351 chocolates.

3) No - could be 24, 25, or 26 - we don't know which.

4) The host was A if 24 people attended, or M if 25 attended, or Z if 26 attended.