Which excerpt from “two kinds” shows that the mother knows how her daughter can become a prodigy?

I'd say that the third sentence, "We'd watch Shirley's old movies on TV as though they were training films" shows that the mother knows "how" her daughter can become a prodigy.

The first sentence just compares the girl to Shirley Temple, but doesn't show that the mother knows how the daughter can become a prodigy, just that she could be.

And the second sentence is just a description of their dining habits.

The last sentence is a very general statement that does not indicate that the mother knows how the child could possibly become a prodigy. Therefore, the second sentence is correct because it shows them watching the Shirley Temple videos like "training videos", which would help the girl become a prodigy.

I hope this helped.