Answer:
B. Mrs. McManus knows a great deal about cars, and she will be glad to show you how to change a flat tire.
Explanation:
In this situation, the purpose of 'and' is to connect 2 independent clauses, 2 sentences which don't depend on each other and can both be used separately as complete sentences:
Mrs. McManus knows a great deal about cars.
She will be glad to show you how to change a flat tire.
Usually, a comma comes before the conjunction 'and' in such situations.