Which of the following were methods used by labor unions to secure better working conditions?

collective bargaining

piecework

strikes

Really its all of the above, but if its multiple choice and thats not an option I think strikes would be the best answer.

Answer: strikes

Explanation:

The labor movement in the United States grew out of the need to protect the common interest of workers.

To get their demands, the unions rely on various strategies, like strikes:

Workers refuse to return to work until their demands are fulfilled. The company comes to a standstill and consent to granting the union's demands.