Answer: the grapevine
Step-by-step explanation:
The options are:
a.social media
b.the company newsletter
c.the grapevine
d.upward communication
Since we are informed that even though the information about recent happenings in the company is confidential, that every employee in the company knows about what's happening and is anticipating major changes in the firm in the near future, we can say this is an informal means of communication which is also referred to as a grapevine.
Other options given in the question are wrong as they don't fit in into the question. Therefore, the correct answer is "grapevine".