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: 
*Prolonger* is more like to append something at the end of an existing object while *allonger* can also mean to stretch something to increase its length.

Both *prolonger* and *allonger* are then possible among the four suggested choices but for an essay, I'd rather write :

> 
> *Elle a essayé d'étoffer son article.* (add more substance to it)
> 
> 
>
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*.