Correct Answer - Option 1 : for
The correct answer is For.
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Grateful For: Grateful is followed by "for" when referring to the thing that makes one grateful (the gift, etc).
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Ex: I am grateful for your lovely gesture.
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Grateful to: when referring to the agent that provided it.
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Ex: I am grateful to my parents for their support.
- Here, grateful referring to the thing "Kindness". So, we used preposition for after grateful.
- Hence, option 1 is the correct answer,
The correct sentence is: They are very grateful for your kindness.
Let's understand the usage of other prepositions:
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With: It is used to describe being together, an instrument, containing. Sometimes it indicates someone's opinion and feeling.
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Ex: He is working with me.
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Towards: It is used to indicate the movement of something which gets closer to anything/object.
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Ex: The dog is walking towards me.