Parentheses to indicate the word can be singular or plural (APA style) The pilot for the study involved two props, one for the participant to keep and another to give to someone else.

The study involved three props, one for the participant to keep and two to give to two other people.

When discussing the study in general, do you write


the props (assumption is that the plural subsumes the singular)
the prop(s)
the prop or props
any of the above, author's choice, just be consistent
Stylistically, it is recommended to choose the most clear and unambiguous terminology, to minimize the risk of a reader misunderstanding. By this metric,


  the props


is a poor choice, because the reader might have forgotten that there might only be one prop, and your writing fails to correct their misunderstanding. I suggest


  the prop(s)


because it is common and standard terminology, unambiguous, and less clunky than "prop or props", which tends to interrupt the flow of the sentence.