Explanation:
The suitable adjective to fill in the blank is option 'c' - kind.

Reasoning:
To determine the appropriate adjective, we need to understand the context of the sentence. The sentence is expressing appreciation for someone offering help.

Definition of the adjectives:
Let's define each adjective to understand why 'kind' is the correct choice:
- Rude: behaving in a disrespectful or impolite manner.
- Horrible: causing or likely to cause horror; shocking or terrifying.
- Kind: having or showing a friendly, generous, and considerate nature.
- Terrible: extremely bad or serious.

Explanation of incorrect options:
- Rude: This adjective implies that the person's action was disrespectful or impolite. However, the sentence is expressing gratitude, so 'rude' would be the opposite of the intended meaning.
- Horrible: This adjective suggests that the person's action was shocking or terrifying. Again, this doesn't align with the positive sentiment expressed in the sentence.
- Terrible: Similar to 'horrible,' this adjective conveys a negative connotation. It implies that the person's action was extremely bad or serious, which contradicts the gratitude expressed in the sentence.

Explanation of the correct option:
- Kind: This adjective perfectly fits the context of the sentence. It implies that the person's action of offering help was friendly, generous, and considerate. It aligns with the positive sentiment expressed, as the speaker appreciates the help given.

Therefore, the correct adjective to fill in the blank is 'kind.'

People behave impolitely