The correct answer is: The metaphor reflects the speaker's negative attitude toward growing older.
The bolded phrase "portentous, menacing road" is a metaphor that compares the speaker's journey into their thirties to a road that is ominous and threatening. This figurative language conveys a negative attitude toward growing older and suggests that the speaker is apprehensive about what lies ahead. The use of the word "portentous" suggests that the speaker sees this new decade as being filled with foreboding or a sense of impending doom, while the word "menacing" suggests that the road ahead is dangerous or threatening. Therefore, the metaphor helps to convey the speaker's negative outlook on their birthday and the passage of time.