The correct answer is:     [D]:
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                     →   " redwood tree forests " .
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  <u>Note</u>:  Let us consider the other answer choices.
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Choice:  [A]:  "garden flowering plants" ;  is incorrect  — since these are not generally considered to be part of a true or traditional "ecosystem" ; and especially since "garden flowering plants" are, in fact, "flowering plants" —  which means that they are "angiosperms" —  <em><u>Not</u></em>  "<em><u>gymnosperms</u></em><em> </em>". 
       →  Rule out:  " Choice:  [A] " . 
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Choice:  [B]:  " ferns and liverworts " ; is incorrect — since <em><u>both</u></em>  ferns — as well as liverworts —<u><em> are Not  </em></u><u>"</u><u><em>gymnosperms </em></u>".   
           Liverworts are "bryophytes" {which lack true vascular tissue] ; and ferns are "pterophytes" ["seedless vascular plants"]. 
       →  Rule out:  " Choice:  [B] " . 
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Choice:  [C]:  " mosses and horsetails "; is incorrect ;  since <em><u>both</u></em> mosses — as well as horsetails — <u><em>are Not  </em></u><u>"</u><u><em>gymnosperms </em></u>".   Mosses are "bryophytes" (see aforementioned) ; and horsetails are "non-seed bearing plants" that spread vegetatively [underground; by asexual reproduction — <em><u>and Not by seed</u></em><em> .</em>].
       →  Rule out:  " Choice:  [C] " . 
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Choice:  [D]:  " redwood tree forests "  ; 
                —  is the only answer choice provided that is a "gymnosperm".     
              Furthermore, the "redwood tree" is a conifer (i.e. "coniferous; or, "cone-bearing" plant} that reproduces through seeds spread by "cone") — and all "conifers" are a type of "gymnosperm."   
             Furthermore, "redwood trees" grow large and tall, and redwood forest communities within an ecosystem; due to there size and presence, are:  "<em><u>known to be important in maintaining the balance of carbon in an ecosystem</u></em>." 
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  →  As such, the correct answer is: 
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  →  Answer choice:  [D]:  " redwood tree forests " .
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Hope this helps!
     Best wishes to you in your academic endeavors
            — and within the "Brainly" community!
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