Le gusta sounds good with the sentence.
B. tu piensas
c and d are talking to more than one person
a is talking about one person but it is refering he or she, el o ella
b is for you, tu
Si Antonio estuviera en la fiesta, el hablaria con Juan.
This sentence means - If Antonio had gone to the party, he would have talked to Juan.
It is a hypothetical sentence, meaning that it didn't happen but states what could have happened had he decided to go to the party.
I believe that it would be B (reloj) since it is talking about how there is a window, a door and then a clock in a classroom.
It is also giving you the masculine singular form indicating that it can only be reloj.
