Question 1
yo pongo una en el sobre.

a.) sello
b.) estampilla
c.) correo

question 2
nosotros la oficina.

a.) anduvimos
b.) pusimos
c.) supimos

question 3
yo carta a mi amiga en nicaragua.

a.) puse
b.) anduve
c.) mandé

question 4
yo sobre en el buzón.

a.) pude
b.) estuve
c.) puse

1. Yo pongo una B. estampilla en el sobre.
First of all, the article is UNA which means you need a feminine noun, and the only feminine noun among your options is estampilla (stamp). The sentence means - I put a stamp on the envelope. Sello is a seal, and correo is mail. So the correct answer is B.

2. Nosotros A. anduvimos a la oficina.
Anduvimos is the preterite form of the verb andar, which means to work. So the entire sentence means - We work in an office. Pusimos is the preterite form of poner (which means to put) and supimos is the preterite form of saber (which means to know) so as you can see, B and C don't fit.

3. Yo C. mandé la carta a mi amiga en Nicaragua.
Mande is the preterite form of the verb mandar, which means to send. So the sentence means - I sent a card to my friend in Nicaragua. Puse comes from poner and anduve comes from andar (both of which I have already explained) so you can see they don't fit this example.

4. Yo C. puse el sobre en el buzon.
Puse is the preterite form of the verb poner, which means to put. So this sentence means - I put the envelope in a mailbox. Pude is preterite of poder, which means to be able to, and estuve is preterite for estar, which means to be. 

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