The pronunciation [plis] for 'police' is an example of:

The pronunciation [plis] for 'police' involves the omission of the initial unstressed vowel sound (the 'o' in 'police', which is typically pronounced as a schwa /ə/ or /oʊ/). This phenomenon, where a sound (especially an unstressed vowel or syllable) is dropped in speech, is called elision.

• A. Linking of /j/: This involves adding a /j/ sound between words, which is not happening here.
• B. Intrusion R: This involves adding an /r/ sound where there isn't one in the spelling, which is not happening here.
• C. Assimilation of consonants: This involves a consonant sound becoming more like a neighboring consonant, which is not the primary change here.
• D. Elision of a weak vowel: This accurately describes the dropping of the unstressed initial vowel in 'police' to produce [plis].

The correct option is D.
$\boxed{\text{D}}$