In experiments testing the cocktail party effect, most participants were unable to do any of the following except
a.
detect when the speaker’s voice changed from male to female
b.
identify any phrase associated with the unattended information
c.
detect when the unattended language was changed to german
d.
notice specific changes when the unattended speech was played backward

 C. detect when an unattended language was changed to German.

The cocktail party effect refers to ability of a person to focus on a single speaker or conservation in a noise prone area. This mainly reflects listening ability of the person in a noisy environment. If a listener hears unattended language for a particular time along with noise.The listener may loose interest  in listening and he will not show alertness when language changes to German which could be known language to listener.