Answer : The correct option is, They repel each other.
Explanation :
There are two types of charges which are positive and negative charge.
There are two conditions:
When the two bodies have like charges that means positive-positive or negative-negative then they repel each other.
When the two bodies have unlike charges that means positive-negative then they attract each other.
As per question, when two balloons have a negative charge that means they have like charges. So, they repel each other.
Hence, the correct option is, They repel each other.