Answer:
The card that Cora bought for me was <em><u>the most thoughtful</u></em> one of all.
Explanation:
The words 'of all' suggests three or more things.
The rule says to use the superlative form for three or more things.
The superlative form is either:
> the + adjective + est (for 1- or 2-syllable adjectives -- though there are exceptions)
> the + most + adjective (for multisyllabic adjectives -- though there are exceptions)
