The answer to your question is letter A. Oxygen, Fluorine, Neon
Explanation:
Data
Elements of the 2nd period
A. The three elements of this list are found in the second period.
B. The three elements of this list are found in the third period
C. One element of this list is in the 2nd period, another in the third and the last one in the fourth period.
D. The elements of this list are found in the second group but not only Beryllium is found in the 2nd period.
PH scale is a universal indicator of strongness of base and acid