(I can't understand this, someone help please. This can be referenced from Law of Sales by Hector De Leon book or any business law books)

D borrowed P100,000.00 from C. The obligation is secured by a mortgage on D's land and building. C registered the mortgage with the Register of Deeds. Thereafter, D sold the land and building to B who was not personally aware of the existence of the mortgage at the time of the sale since only the photocopy of the transfer certificate of title which did not yet contain the annotation of the mortgage was shown to the buyer. It was only when B went to the Registry of Deeds to register the sale of the land and building that he learned of the mortgage. Based on the facts, please choose the correct letter of the statement below and explain your answer.

a. C can collect from D and if D cannot pay, C can foreclose the mortgage although the land and building are now owned by X.

b. C can collect from D, but if D cannot pay, C cannot foreclose the mortgage because X was not aware of the existence of the mortgage at the time he bought the land and building.

c. C cannot collect from D. He can only go after the mortgage which was given as security.

d. C cannot foreclose the mortgage because X was not a party thereto.