Unlike most gynodioecious plants, the male and female Catasetum orchid are very easy to tell apart.
(A) Unlike most gynodioecious plants, the male and female Catasetum orchid are very easy to tell apart.
“orchid” is singular and thus disagrees with “are”. We need “rchids” as in E.
(B) Unlike most gynodioecious plants, the gender of the Catasetum orchid is very easy to distinguish.
“unlike” illogically compares plants with gender.
(C) Unlike those in most gynodioecious plants, the male and female Catasetum orchids are very easy to distinguish.
“those” refers to “catasetum orchids” and thus creates illogical comparison. “those” cannot refer to “male and female” because they are adjectives, not nouns.
(D) It is very easy, unlike in most gynodioecious plants, to tell the male and female Catasetum orchid apart.
“unlike” is a noun modifier and thus cannot be followed by a presposition “in”.
(E) Catasetum orchids are unlike most species of gynodioecious plants in that it is very easy to tell the male and female apart.
The correct answer: “in that” is correctly used to give a reason.
Hence E