The organisms which show difficulty in their structure are Multicellular organisms.
Explanation:
Organisms that comprise of more than single cell are called multicellular organisms but even ‘single-celled’ organisms are quite complex. Multicellular organisms consists of multiple cells each of which is specialised to perform a particular function. These cells unite to form tissues and then organs like heart, lungs, etc. Each of these organs has a specific function to perform which helps in effective working of the body.