What is the gender of Armin in Shingeki no Kyojin? One of my friends read an article which claimed that Armin Arlert of Attack on Titan is female. Can anyone confirm or deny this?
This article included the following statement:

Q: Out of the group of female characters in the 104th Trainee Squad, who did Mikasa get along with the most?
A: Armin

This related article further states the following:

Isayama has revealed that Armin is a female character.  Now this is a huge surprise for Shingeki no Kyojin fans.  Everyone thought that Armin was always a boy but it looks like she was a girl.

Finally, here's another article discussing the possibility of Armin being female.
Armin is a boy.
There are some reasons worth consideration:

His appearance is male. This isn't always telltale, due to traps.
Armin uses distinctly male language in Japanese. (He refers to himself as 僕【ぼく】 (boku), which is a male pronoun, fairly rarely used by females unless they are traps.)
The Shingeki no Kyojin Wiki refers to him as male.
Armin is a boy's name. (One source, but there's many.)
He is voiced by a male in the English dub. Though he is voiced by a female in Japanese, this is common for young or weak boys (Shinji Ikari, Edward Elric, etc.). However, a male voicing a female character is exceedingly rare, and I doubt the mangaka would have allowed it.

In the article you are referring to, the answers to the questions are actually just for humorous purposes, and should not be regarded canonically. For example:

Q: Mike smells all kinds of stuff, how does a titan smell?
A: They smell like a spa-town.

After this article, Armin was later confirmed to be male by the mangaka himself at a public event found in this Japanese video at 3:09 (thanks @OshinoShinobu):

Q: アルミンの性別はどっちですか？
What is Armin's sex?
A: 男です。
Male.