Four words have been given out of which three are alike in some manner and one is different. Select the one that is different.

Mellifluous, perspicacious, and discerning all describe positive mental qualities.

Mellifluous refers to something that is pleasing to the ear (sound).

Perspicacious means having a sharp intellect or good judgment.

Discerning means having or showing good judgment.

Ineffable, on the other hand, means something that cannot be expressed in words.

Therefore, while the other three words describe positive traits, ineffable describes something beyond words.