Yep, you have to use the ser verb to fill in the blanks.. ser, sera, seran, etc
12. Tanta, como
13. Tan, como
14. Tantas, como
15. Tan, como
Conjugations of verbs for the formal "you" in Spanish are the same as for the subjects el and ella; in present tense, they usually end in -a.
The option you have checked is in past tense ("I bought this"); the present tense conjugation is compra.
Usted compra esto.
Again, the conjugation of verbs for the formal "you" in present tense almost always end in the letter "a"; the correct conjugation is estudia.
Usted estudia <span>matemáticas.</span>
Gustan A is the correct answer
Sources: Me