Robert and sue ellen concentrated on the dead worm on the dissection tray. [they] used tweezers to pick through innards looking for the heart.

which, if any, correction needs to be made to the underlined pronoun in the sentence above?

question 3 options:

he

she

he or she

no change is necessary.

I am pretty sure that you do not need to change something to make this sentence look better. Gramatically everything is totaly OK, so the last option - that's your answer. As you can see,  '[They] used tweezers to pick through innards looking for the heart' the previous tense is being used there, so any pronoun would be possible to use and that also would be proper.