The negation of the propositions are as below,
A. Jennifer and Teja are not friends.
B. There are more than 13 items in a baker's dozen OR less than 13 items in a baker's dozen.
(or)
There are not 13 items in the baker's dozen.
C. Abby did not send more than 100 text messages everyday.
(or)
Abby sent less than or equal to 100 text messages every day.
D. 121 is not a perfect square.