Question

Identify where a comma should be placed by choosing the word after which the comma should be placed. if there is no error, mark the answer for “no error.” abbey is most interested in learning how to mount her horse, for she already knows several things about dressage

Answer 1

The answer is: 
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"no error".
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The comma is correctly placed before the conjunction "for"; and in this instance in which the word, "for", is used, a comma is necessary.  No other part of the sentence needs—or requires—a comma, and there are no commas that are placed unnecessarily.
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As a side note, I would recommend that the first letter of the first word in the sentence: "Abbey", be capitalized, because:
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1) It is the first word at the beginning of the sentence; and:
2) It is someone's name, which is a proper noun.
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I also recommend placing a "period" at the end of the sentence.
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To read:
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Abbey is most interested in learning how to mount her horse, for she already knows several things about dressage.
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