Answer: c. Margaret may claim her mother as a dependent if her sister agrees in a multiple support agreement.
Step-by-step explanation:
For Margaret to claim her mother as a dependant, she would have to be providing more than 50% of her mother's support during the year. Margaret alone provides 40% so cannot claim her mother as a dependant.
If her Margaret and her sister can however agree to a multiple support agreement, Margaret may claim her mother as a dependent and their join support of her would pass the 50% margin required.