Final answer:
In English, [f] and [v] are two contrastive phonemes, which are distinct units of sound that distinguish words from one another, such as 'leaf' and 'leave'.
Step-by-step explanation:
If [f] and [v] are contrastive in English, it means that they are two different phonemes in English. Phonemes are the smallest units of sound in a language that can distinguish one word from another. [f] and [v] represent two distinct sounds, and the difference between them can change the meaning of words. For example, 'leaf' ends with an [f] sound, while 'leave' ends with a [v] sound, illustrating how these phonemes can alter the word's meaning.