Tara raises horses and promised to give her best friend's niece a pony. Tara advised the girl's father, Jon, that he would need to enclose a section of his property to keep the pony safe and recommended a specific type of fencing. Jon bought the recommended materials and had the fence built on his property. Tara got in a fight with her best friend and now refuses to give Jon's daughter the pony. Tara will be liable to Jon for the cost of fencing his property based on the theory of 

a. Quasi contract

b. Substantial performance

c. Unjust enrichment

d. Promissory estoppel