What's a chosen plaintext attack?

It is an attack when an attacker is able to obtain ciphertext for arbitrary plaintext. They then analyze the encryption of the plaintext to acquire the corresponding ciphertext.

It's an attack when an attacker has the ciphertext of several messages encrypted with the same algorithm. They then need to find the key.

It's an attack when an attacker has the plaintext and the encryption algorithm, and determines what the ciphertext is.

It's when an attacker injects plaintext into an encryption algorithm in order to discover a buffer overflow.

Explanation

An example of chosen-plaintext attack is when an attacker sends a target socially engineered plaintext that encourages the target to encrypt it. When the attacker created the plaintext that was sent to the target, they can cryptanalyze the cipher. (EDIT: Poor wording, arbitrary implies random which is an opposite of chosen.)