a marriage singular subject ; singular verb each other
and marriages plural subject ; plural one another
only option B & C are left
c. marriages are
mostly successful when they are founded on open communication, mutual trust, and respect for one another ; use of mostly is wrong as its adverbial and is not required for a condition
b. a marriage is most successful when founded on open communication, mutual trust, and respect for each othercorrect option B
TBT
According to most relationship counselors, a marriage is most successful when it is founded on open communication, mutual trust, and respect for one another.
a. a marriage is most successful when it is founded on open communication, mutual trust, and respect for one another
b. a marriage is most successful when founded on open communication, mutual trust, and respect for each other
c. marriages are mostly successful when they are founded on open communication, mutual trust, and respect for one another
d. marriages are most successful when their foundation is open communication, mutual trust, and respect for each other
e. a marriage founded on open communication, mutual trust, and respect for each other are mostly successful