I can't make a choice between the options you have given because I don't understand them. I will try to explain the difference between phonemes and allophones.
A phoneme, as you correctly say, is the smallest contrastive unit in the sound system of a certain language. A phoneme is the smallest part of an utterance that cannot be changed if we want to retain the meaning. For example,
is a correct English utterance. Its phonemes are: /aɪ/, /s/, /i:/.
If we change the second phoneme to the phoneme /p/, we will get
which has a different meaning.
If we change the first phoneme in the original utterance to the phoneme /p/, we will get
which doesn't mean anything.
This explains that changing phonemes changes the meaning. Changing allophones, on the other hand, doesn't change the meaning. Let's take the original utterance again.
When a British person says it, they are more likely to pronounce the last phoneme (/i:/) longer than an American would. (It's a generalization. It's only for the sake of explanation.) We denote the British version of the phoneme as [i:]. We denote the American version as [i]. Note the use of square brackets ("[" and "]") instead of slashes ("/"). We change the sybols because we no longer want to talk about phonemes. We want to talk about allophones now and the change of notation helps us distinguish between the two things.
Allophones are realizations of phonemes. Every person pronounces words in their own specific way. The sounds we make when we say I see aren't exactly the same. We "mean" the same sounds (that is phonemes) but in reality we produce different sounds (allophones).
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