In the 3rd and 4th stanza of the poem"Postcard from Kashmir" by Agha Shahid Ali, why does the poet break line into more than...

I assume when
you say "the poet breaks the line into more than one" you mean the poet breaks the
sentence into lines.  An important part of poetry, the line is completely different than a
grammatic sentence.  A full sentence can continue for more than one line, and sometimes more
than one stanza.  The reason for breaking a sentence into lines of poetry can be varied, some of
which you identify in your question.  Poets can break lines for rhythmic or sound effect, as in
a poem with a specific rhyme scheme.  A line can also be broken to achieve a certain shape, as
in concrete poetry, to emphasize a word or phrase, to compare or contrast words, or to force the
reader to speed up or slow down.

Agha Shahid Ali's beautiful little poem,
"Postcard from Kashmir" mimics its subject matter in size and shape.  The poem, only
four stanzas, is like a postcard in that it packs much into a small space.  In this way, you
could argue that #1 is the correct answer, and that the poet is trying to recreate the look and
feel of a postcard.  But as in most poetry, there is no right or wrong answer, only
interpretation.

I actually see a much stronger argument for #2.  Shahid Ali
has broken the line at some very significant places in stanzas 3 and 4 which affect the reading
of the stanza.  In particular in stanza 3, there is a high amount of repetition of phrase
structure: "so brilliant...so clean/so ultramarine...so overexposed."  By breaking the
lines where he does, Shahid Ali draws attention to this repetition, which creates a subtle
rhythm that is almost prayer-like or chant-like in quality.  He also draws attention to the
language of photography by ending stanzas 3 and 4 with the words "overexposed" and
"underdeveloped."  By drawing emphasis to these words, Shahid Ali compares the
permanent quality of a photograph to the impermanence of memory.