"Start" and "commence" are synonyms, just as "end" and "conclude" mean similar things.

"Stranger" and "outsider" are both synonyms, just as "familiar" and "well-known" have similar meanings.

"Mute" and "silent" are both synonyms, just as "loud" and "deafening" have similar meanings.

"Decent" and "polite" are synonyms, just as "improper" and "misbehaving" are similar in meaning.

"Senile" and "decrepit" are synonyms, just as "juvenile" and "youthful" have similar meanings. 

"Autonomous" and "independent" are synonyms, just as "dependent" and "subservient" have similar meanings.

"Panic" and "dismay" are synonyms, just as "composure" and "confidence" have similar meanings. 

A fib is generally a small, insignificant lie; therefore, the bridge sentence for this analogy would be, “Just as a fib is an insignificant or small type of lie, so to a rivulet is a small type of X.” While it might seem too tempting to think a “rivulet” is a small stream, this actually is the case. The “-let” suffix is used to alter its base and express the notion of being “small.” For instance, a “booklet” is literally a small book. (They aren’t just called that by happenstance!) None of the other options capture this relation of “small version to larger or more general type.”

When someone is “elated,” he or she is said to be so happy that he or she is “carried out of himself.” (This is somewhat like the word “ecstasy,” which literally means standing outside of oneself). Elation is thus a state of extreme joy—something at least relatively synonymous with being “thrilled.” Thus, we are looking for a synonym for the word “grin.” Among the options, the only word that is synonymous to “grin” is “beam.” This might seem strange at first, but consider “grin” taken as a verb—“to grin.” Although the word “beam” can also mean something completely different—e.g. the wooden beams in a house—as a verb, it can also mean to smile broadly and openly. Perhaps you have seen the present participle: “A beaming smile.”

"Deciduous" plants are those which lose their leaves annually (as opposed to pine and other evergreen trees that retain their needles throughout the winter and into the spring). The word literally comes from Latin roots meaning to fall downward. The “de-” prefix means down from (as in “descend”) and the “-ciduous” is related to the same and similar forms found in “cadence” and “accident.” Etymology aside, the analogy is one of member-to-class. Something deciduous is a type of arboreal plant. ("Arboreal" meaning pertaining to trees). Thus, something “bodily” is best called something “corporeal.” None of the other options name the general class of bodily things. The word "corpulent," which is close (and clearly related in its roots) means fat—like having a lot of “body.”