To be enamored means to be in love with or to really like something. So which of the following is the correct idiomatic usage?
He was so enamored of/by/with beagles that he volunteered to adopt unwanted beagles from the dog kennel.
Two of the above are appropriate. Indeed, all three prepositions could work, however one of them changes the meaning. Let’s take a look.
If we were to say he was enamored by beagles then we would be saying that beagles are in love of him. Clearly, he really likes beagles and is willing to adopt them (not the other way around.) Also in saying enamored by we are lapsing into the passive tense, something the GMAT mostly—but not always avoids—in correct answers.
As for enamored of/with, either is correct. Enamored of is used more often in academic/literary context and is probably the form GMAT would favor. Enamored with is typically softer on the ears, probably because it is often used colloquially. Nonetheless, it is still correct to say enamored with, as in:
He was so enamored with the GMAT that he purchased every book on the market, missing his favorite TV shows to practice idioms.
While grammatically correct, that sentence probably isn’t too realistic!