Answer: The new meaning of "act" when the prefix "counter-" is added is C) the act of saying something opposite in meaning to something else.
Explanation: A prefix is a morpheme that is attached to a root word in order to give place to a new word. Moreover, prefixes are placed before a root word. In the case of the word "counteract", "counter-" is the prefix and "act" is the root word. Furthermore, as the meaning of the prefix "counter-" is "against something" or "in opposition to something", the meaning of the word "counteract" is to act or to say something against someone else.