The correct answer is B. 'we; subject'. If it helps, try rephrasing the question as a statement.
Jenny and ___ are walking to the soccer fields this afternoon.
The missing word has to be a subject and not a predicate nominative, since 'walking' is an action verb (predicate nominatives are only present in sentences with a linking verb). Therefore, the answer is either A or B. Now, let's substitute both 'us' and 'we' into the sentence and see which one works better.
Jenny and us are walking to the soccer fields this afternoon.
Jenny and we are walking to the soccer fields this afternoon.
If it helps, try using a singular pronoun instead of a plural pronoun, since both sentences sound a bit strange.
Jenny and me are walking to the soccer fields this afternoon.
Jenny and I are walking to the soccer fields this afternoon.
Now it's pretty obvious which one is correct. I think we've all had an English teacher or a Grammar Nazi correct us when we say 'Me and my friend'. 'Me' cannot be correct here, therefore the plural form of 'me', 'us', cannot be correct either.
Therefore, the answer is B. 'we; subject'.
Hope this helps!
