Fill in the blanks with the most appropriate option:-

I am very thankful to you for this …….. of kindness.

act

The most appropriate option to fill in the blanks is act.

The sentence is about the speaker expressing their gratitude for someone's act of kindness. The word "act" is a noun that means "a single instance of doing something". It is the most appropriate word to use in this context, as it describes the specific act of kindness that the speaker is grateful for.

The other options are not as appropriate:

• Thing is a noun that means "an object or entity". It is not as appropriate as "act", as it does not convey the same sense of specificity or intentionality.
• Action is a noun that means "a thing done". It is not as appropriate as "act", as it is more general and can refer to any kind of action, not just an act of kindness.
• Work is a noun that means "activity involving mental or physical effort". It is not as appropriate as "act", as it does not convey the same sense of generosity or compassion.