when should a write use a dash?

a. to indicate an abrupt shift in idea
b. to set off an introductory word or phrase
c. to separate clauses in a compound sentence
d. to separate items in a list

A writer should use a dash in the following situations:

a. to indicate an abrupt shift in idea - when there is a sudden change in thought or topic within a sentence or paragraph.
b. to set off an introductory word or phrase - when introducing a word or phrase at the beginning of a sentence for emphasis or clarification.
d. to separate items in a list - when listing items in a sentence, especially when there is a need to emphasize each individual item.

So the correct options would be a, b, and d.