(b) because she smiled at me
Phrases are a group of words that are a part of a sentence and do not contain both a subject and a verb. In other words, a phrase does not contain a subject doing an action.
Analyzing the given options:
(a) leaving behind the dog
This group of words does not contain a subject even though it contains a verb - leaving. Therefore it is considered to be a phrase.
(b) because she smiled at me
This group of word among other things, contains a subject - a pronoun she - and a verb - smiled. Therefore, it is not considered to be a phrase.
(c) smashing into a fence
This group of words does not contain a subject even though it contains a verb - smashing. Therefore it is considered to be a phrase.
(d) before the first test.
This group of words neither contains a verb or a subject. Therefore it is considered to be a phrase.
(e) pushing the sofa.
This group of words does not contain a subject even though it contains a verb - pushing. Therefore it is considered to be a phrase.
