Question: To express "lengthen an essay", should I write "*prolonger la composition*" or "*allonger la composition*"? I found in the dictionary saying these two words both means to lengthen.

My question is "Elle a essayé de/d' \_\_\_\_\_\_\_\_ son article", given the options to fill in the blank are 
A prolonger
B longer 
C allonger 
D accroître. 

Thank you so much for your help :)

Answer: 
To express "lengthen an essay", I would personally use "rallonger la composition".

According to [Larousse definition](https://www.larousse.fr/dictionnaires/francais/rallonger/66321):

> 
> Accroître la longueur de quelque chose : *Rallonger un article de quelques lignes*.
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> 
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It is a very close synonym of "allonger" (and nowadays, I think you can use both as you wish), but it expresses the idea of adding something. The definition I linked above states:

> 
> Allonger = rendre plus long ou faire paraître plus long.
> 
> 
> Rallonger = rendre plus long **en ajoutant quelque chose**.
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> 
> 

But it's a subtle difference. Back to your question.

**What is the difference between *allonger* and *prolonger*?**

Most of the times, "prolonger" is used to express lenghtening something **in time**, or for an existing **line** (road, train line, etc).

> 
> *Suite à ses performances, nous avons décidé de prolonger sa période d'essai.*
> 
> 
> *Grâce aux récents travaux, la route a été prolongée jusqu'à la ville voisine.*
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> 
> 

I would use "allonger" when I'm talking about something more abstract.

**Please note** that "allonger" also has other meanings, the main ones being:

* To lay someone down: *J'ai allongé la victime par terre pour qu'elle se sente mieux.*
* To extend / stretch a body part: *J'ai besoin d'allonger le bras pour atteindre cette boîte dans mon placard.*

All the meanings are listed in the [Larousse definition](https://www.larousse.fr/dictionnaires/francais/allonger/2416) for "allonger".
If what you mean is that she padded her essay with less relevant stuff to make it longer, then *allonger* is perfect. It is used in cooking, where *allonger une sauce* means to dilute it with water. It can be used figuratively for productions of the mind such as an essay or an article and it would be understood as a slight criticism. Otherwise, a neutral way of expressing the idea would be *rallonger l'article*.