Let's go over the translation of each phrase:
A: la llamada telefónica a la compaña = the phone call to the company
B: ir a la pagina de reservas por Internet = go to the online booking page
C: visitar la agencia de viaje = visit the travel agency
D: ir a la oficina de turismo para obtener información = go to the tourist office for information
We have our translations, now let's go back to the sentence:
Emily is booking plane tickets online.
Which makes most sense?
A - she's not making a phone call. This is wrong!
B - yes, she is on the online booking page. This is correct!
C - no, she's not visiting a travel agency. This is wrong!
D - she isn't going anywhere for information. This is wrong!
So..
The correct answer would be B, ir a la pagina de reservas por Internet.
Hope this helps!
- Alice