Answer:
A. That the weather in Scotland is usually very bad
Step-by-step explanation:
The idea that Shakespeare tries to introduce is that the weather in Scotland is usually very bad.
The scene opens with the three witches of which the first witch asks when next they would meet, in thunder, lightning or rain. This is a first glimpse that the weather is usually not stable.
Next, they all reply "foul is fair and fair is foul, hover through the fog and filthy air". This indicates that the weather is usually foggy and the air is filthy.