Implicit biases may be subconscious, however they are reliant upon pre-existing networks of thoughts and associations and clusters of knowledge.
At the end of the third paragraph of this passage, they state that a "shorter response time for a particular combination of image and attribute indicates a stronger link in the participant's mind."
So they see images which connect to their schemas instantaneously and make their responses as soon as possible.
You may construct schemas consciously, but you certainly rely on them when making split-second unconscious decisions.
Plus, none of the other choices make sense.