Select the word-pair that best represents a similar relationship to the one expressed in the pair of words given below.
(The words must be considered as meaningful English words and must not be related to each other based on the number of letters/number of consonants/vowels in the word)
Pleasure : Pain

The correct answer is Gentle : Brutal.

Step 1: Analyze the relationship in the given pair
The given pair of words is Pleasure : Pain.
'Pleasure' and 'Pain' are direct antonyms (words with opposite meanings).

Step 2: Evaluate each option to find a matching relationship

1. Angry : Furious
'Furious' means extremely angry. These words are synonyms, not antonyms.

2. Smart : Intelligent
'Smart' and 'Intelligent' mean the same thing. These words are synonyms, not antonyms.

3. Fast : Quick
'Fast' and 'Quick' both describe high speed. These words are synonyms, not antonyms.

4. Gentle : Brutal
'Gentle' means mild, kind, or tender, whereas 'Brutal' means harsh, violent, or cruel. These words are direct antonyms, which matches the relationship expressed in Pleasure : Pain.

Therefore, the word-pair that best represents a similar relationship is Gentle : Brutal.