criteria to select 180 degree as a c
why we choose -180 degree as a critical point for bode plot? i want
  the explanation for why we choose the factor 180 degree


Stability when using negative feedback is an important thing in engineering and a simple example is an inverting op-amp; it naturally inverts (shifts by 180 degrees) and so any feedback acts to cancel the input signal to a lesser or greater extent i.e. that feedback is "negative" and stabilizing.

However, op-amps are not perfect and, at high frequencies, some op-amps shift there response so that negative feedback eventually becomes positive feedback i.e. another 180 degrees is added to the natural phase shift at lower frequencies.

This turns an amplifier into an oscillator and is one way of looking at "instability".

This is the open loop gain (before feedback is applied) for a typical op-amp (OP-77): -



If you study the graph and look at the frequency where the gain drops to unity (about 400 kHz) you can see that the op-amp (as an inverter) hasn't added enough phase shift to bring it to 180 degrees - this means that it will always be stable when simple resistor feedback is used. This isn't the case for some op-amps though.