how to tell the difference between "introduction","instruction" and "direction"?

Instructions for how to take medicine are listed on the bottle, usually on the back.

If you don't know this, the sentence could be confusing.

There is always a heading on the label:
Directions For Use.

It should be 'directions', which you give in your INTRODUCTORY sentence, but do not list in your alternatives.

'directions' might be as simple as "Take 2 tablets every 6 hours".

'instructions' = (often detailed) information about what is to be done, or how something should be done; with the added meaning of 'direction', this has the sense of an order!
I have never see the word 'Instructions' on any kind of medicine; but maybe if I had ever taken MAO Inhibitors or some such, that can have fatal results if certain guidelines are not followed, then, maybe, they might say "Instructions".

So: if 'directions' was NOT one of the alternatives, I consider the question is at fault, and I would HAVE TO give the answer as a given alternative, "Instructions".

-How can I take the medicine, sir?
--Just follow the __ on the bottom.
C instructions

In the US, you could also say: Just follow the DIRECTIONS on the bottle.
(The directions for the medication would be printed on a label pasted on the bottle ( or vial).

Introduction is an initial familiarization. Think of it like an introduction to a person you don't know.
Instruction usually means to teach although saying "Follow the instructions" is not incorrect.
Directions are rules to follow exactly. If I give you directions to the store, I will tell you exactly what to do. "Follow the directions" is the best answer here.

In the UK instructions and directions are interchangeable, at least regarding medicine.

The explanation tells you what the drugs are and what they do.
Message is far too vague, it could refer to any communication.

Same in the US. I like "direction" if I had to pick one but I wouldn't call either incorrect.