Correct option is C
Option (c) is the correct meaning of the given idiom.
Green horn: This idiom is used to refer to a person who is new to a particular activity, field, or job and therefore lacks experience or skill. (अनुभवहीन व्यक्ति)
Example: Rohan is just a green horn in programming, but he learns quickly.
Other related idioms and their meanings:
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Wet behind the ears: Lacking experience
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New kid on the block: Someone new to a place or organization
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Still learning the ropes: Still learning how things work