Who was/is the greatest Afghanistani scientist? What was her/his contribution to the scientific advancement of mankind?
Avicenna made important contributions to astronomy, geometry and as a physician, medicine. He was born in what is now known as Afghanistan.
You’d need to define “Afghanistan”. Do you mean the current country? Or the place where this country sits? There’s been a lot of history in that place…
Let me guess… school asignment, right? 
Correction: Avicenna was born near Bukhara in what is now Uzbekistan. He worked in Iran.
Another correction: The adjective for a person from Afghanistan is simply “Afghan,” not “Afghanistani.”
The answer to the OP is Abu Rayhan al-Biruni (c. 973-c. 1050). He was born in Khwarezm, in what is now Qaraqalpaqistan, in western Uzkbekistan, but he lived and worked in Afghanistan at the court of Sultan Mahmud of Ghazna. He is considered not only the greatest scientist in Afghanistan’s history, but probably the greatest Islamic scientist of all time.
I think you’ll score some extra points with the teacher if you reported on the greatest “Afghan” scientist, instead.
Afghanistani:
Noun 1. Afghanistani - a native or inhabitant of Afghanistan
Adj. 1. Afghanistani - of or relating to or characteristic of Afghanistan or its people
Afghan, Afghani
A simple Google search of would have confirmed your use of “Afghanistani”! Wish I could suggest an answer for an Afghani scientist …
What dictionary did you pull that “Afghanistani” word from? Or did you make it up? I looked in the American Heritage Dictionary, 4th ed., and it had both “Afghan” and “Afghani” as the adjective for that country, but “Afghanistani” was not an entry. Just because you can find a word somewhere on the web using Google doesn’t make it standard English. You can find an awful lot of substandard English out there. Speaking as a linguist working with South Asian languages, including Dari and Urdu (and I work with several guys from Afghanistan), people over there do not use the form “Afghanistani” to describe themselves. They call themselves “Afghani” in their native languages, and the usual form in English is “Afghan.”
Anyway, follow the link I posted about al-Biruni, and I suggest going to the library and looking up Abu Rayhan al-Biruni in the Encyclopædia Britannica.
So you would challenge this word in a Scrabble game based on the American Heritage Dictionary?
Nope! Lots of substandard usages get legitimized out there.
Just because “Ask Jeeves” and on-line free dictionaries might list a word, doesn’t make the word proper, currently acceptable, nor even correct.
The word “Afghanistani” turns up once in print in an online database search of 50+ typical US newspapers from 1900-1990.
If a majority of the population would use the word, it might come into common use. But they don’t and it hasn’t.
YDMV.