Got it, thank you!
So this question isn't asking where an abortion could possibly be induced to happen - it's testing if you know where a blastocyst must implant to avoid a (spontaneous) abortion. The blastocyst has to implant into the endometrium for a successful pregnancy to develop. If it doesn't implant there (i.e. implants anywhere but the endometrium), then as the answer explanation points out, you'd end up with an ectoptic pregnancy which results in spontaneous abortion.
The use of 'aborted pregnancy' may have thrown you off here because we often associate the word abortion with the intentional termination of pregnancy, as that's mostly how the word is used. But abortion could also refer to a spontaneous abortion, which is basically the failure for the embryo to develop at some stage of development. A easier way to phrase this question would have been "For an embryo to successfully develop the blastocyst must implant in the...". Note that the original question doesn't imply the embryo is guaranteed to successfully develop once it implants in the endometrium, just that the embryo is almost guaranteed to fail if it doesn't.
