This is the 692nd most frequent Italian word.
sullo
"Sullo" means "on the" or "upon the" in English.
Here, 'sullo' is used as a contraction combining 'su' (on) and 'lo' (the definite article for masculine singular nouns starting with a z or s+consonant), forming 'sullo'.
Il libro è sullo scaffale.
The book is on the shelf.
In this sentence, 'sullo' acts as a combination of 'su' (about/on) and 'lo' (the definite article), indicating the topic of the discussion, 'lo sviluppo'.
Stiamo discutendo sullo sviluppo sostenibile.
We are discussing sustainable development.
Here, 'sullo' merges 'su' (on/based on) and 'lo' (the definite article), contextualizing the subject of reliance 'lo studio'.