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.