Sheldon Plankton sang a cheerful song to his sick armadillo.
Explanation:
The direct object is the receiver of the action represented by the verb (the predicate). To identify the direct object, you can find the verb and ask<em> what?/ whom?</em>For example,
He ate an apple.
He ate <em>what? </em>
An apple. - direct object
We can do the same here:
Sheldon Plankton sang a cheerful song to his sick armadillo.