You can easily spot a passive structure by looking for the verb "to be" (am, are, is, was, were, have been, has been, will have been, would have been, etc.) together with the participle (the third verb form, e.g. seen, gone, shown, eaten, written, etc.).
In this case, your passive structures are the following:
a) is used
e) are required to express
In each case you have the verb "to be" in combination with the participle: is used, are required.
The others are not a passive structure, because they either use an active verb (makes, keeps, helps) or they use the verb "to be" with an adjective (are concise, is effective).
I hope this helps you identify the passive structure more easily in the future!