Which practice was common among modernist poets?
a using experimental techniques
b employing traditional forms
c following strict meter
d writing in formal language
i think its a

The practice which was common among modernist poets was A. using experimental techniques. 
They defied every single characteristic of previous literary eras, especially B, D, and D, which is why these are incorrect answers.
Modernism, as the term itself says, was all about something new and never seen before, which is why they used many experimental techniques.