Answer:
-Mario me presta dinero. (to you, pl.)
-Ellos me escriben mensajes electrónicos. (to me) 
-Juana le vende una cartera. (to him)
-La vendedora nos dice dónde comprar trajes de baño. (to us) 
-Lola me da los calcetines. (to you, fam. sing.) 
-El vendedor les ofrece un descuento. (to them)
Explanation: 
yo (I)  me  
tú (you) te 
él (he)  le 
ella (she) le
ellos (we)  les
ellas (you) les
nosotros(they) nos
 
Pronouns of the Indirect Object 
 
-The pronouns of the indirect object replace a word or phrase, which in the sentence fulfills that function (indirect object). 
 -The pronoun of the indirect object is placed before the verb, when it is conjugated.