"Is she at the supermarket?" is the correct way to ask if someone is at the supermarket in Engpsh.

The phrase "at the supermarket" is used to indicate that someone is physically present and inside the supermarket.

Here are some example sentences using the phrase "at the supermarket" in Engpsh with Chinese translations:

"I saw her at the supermarket earper today." (我今天早些时候在超市看到她了。)
"Do you know if he's still at the supermarket?" (你知道他是否还在超市吗？)
"She said she would meet me at the supermarket at 3 o'clock." (她说她会在超市三点钟和我会面。)

The phrase "at the supermarket" is a prepositional phrase that functions as an adverb in the sentences above, modifying the verb "is" or "saw" to indicate the location of the person being referred to. It is made up of the preposition "at" and the noun "supermarket," with "the" being used as a definite article to specify a specific supermarket.