We use "at" with specific times of the day. "At" is correct because evening is considered a specific time period.
We say: at night, at noon, at midnight, at dawn, at dusk, at sunset, at sunrise.
2. I brush my teeth _____ a day. 🦷
Two
Twice
"Twice" is the correct adverb of frequency meaning "two times".
"Two" is just a number and needs to be followed by "times" to make sense (two times).
3. I can't find my keys. I don't know where ____ are. 🔑
it
them
they
"They" is correct because we need a subject pronoun here. "Keys" is plural, so we use "they".
"It" is singular and "them" is an object pronoun, not a subject pronoun.