Action potentials propagate as invariant (stereotyped) signals. what does the word "invariant" in this statement mean?
a. action potential are cool
b. action potentials are all or nothing signals
c. action potentials vary depending on the stimulus
d. action potentials are sometimes fast and sometimes slow

The correct answer to the question: What does the word "invariant" in this statement mean?, would be: B: Action potentials are all or nothing signals.

Explanation:

An action potential is almost like the way that cells, especially neural cells communicate and contact one another. In order for this potential to happen, certain ionic changes must happen both in and outside of cell membranes, which charges the cell until it surpasses a certain threshold and fires out signals, both in, and out of it. One characteristic of action potentials, however, is that they are an "all or nothing" type, which means, that once initiated, an action potential will reach threshold and then fire, whatever else happens. Their intensity doesn´t change and once started it cannot be stopped. This is what the word invariant means, these signals will happen once the stimulus has initiated them, but they will not change in nature, or course, regardless of further stimulus. What stimulus does is no initiate the same potential but in more cells than the original one, but each individual cell will undergo action potential activities with, or without, more, or less, stimulus.