What types of organisms show increased complexity of their structure?

Multicellular organisms.

Explanation:

They have many cells all working in unison.

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.

there are so many examples for that in different areas, like biology experiment carried out in our lab recently.

here's one link:

d. planting trees

the trees take in carbon to make oxygen.