Last week the boys . . . . . . . . magic show.

The correct answer is B: saw.
This question tests your understanding of past simple tense.
The phrase "Last week" indicates that the action happened in the past.
Let's look at why the other options are incorrect:
A: will see - This is future tense and doesn't match "Last week."
C: have seen - This is present perfect tense, which indicates an action that started in the past and continues to the present or has relevance to the present, but it doesn't fit with a specific past time like "Last week."
Therefore, "saw" is the correct past simple tense verb to use in this sentence to indicate what the boys did last week.