Answer:
The correct answer is option b. dijimos.
So, the phrase is: Mis amigos y yo dijimos que fue aburrido.
Step-by-step explanation:
All the options presented are conjugated in the past tense correctly, but not all are agreeing with the subject. In Spanish conjugation changes depending not only on the tense but also on the gramatical number and person used. The subject is the one that addresses both, and for the sake of the agreement of the whole sentence the verb conjugation has to be coherent with it.
Note that the phrase is indicating that My friends and I ("Mis amigos y yo") said that is was boring, which refers to the plural first person (we). So, only option b. dijimos matches this criterion, as is the conjugation of the Spanish verb "decir" in the plural first person (we).
Option a. dijo is not correct because is conjugated in the singular third person (she/he). Option c. dijeron is neither the correct answer for it is conjugated in the plural third person (they). And option d. dijiste is incorrect because is using the singular second person (you).