Unstable output in VT pin of HT12D decoder
The VT pin only goes high when a valid transmission is being received. If the received signal is noisy (which it will be unless the transmitter and receiver are close to each other) VT will randomly drop out during transmission. And you should not be transmitting continuously anyway, so if you want the pump to stay on then you need a latched signal.


  power on state is active low so whenever the circuit is powered off
  and then turned ON again it automatically gives output in D8 pin and
  in turn drives the water pump.


The obvious solution is to make the pump turn on when D8 is high, not low. However transmitters using the H-12E usually have buttons that produce a low output when pressed, in which case you would have to modify the transmitter to make the button produce a high signal. Modifying the transmitter may not be an option, so...

If one of the other data outputs is not being used you could connect IC1a to it, then when a transmission is received that data output will go high (and stay high) because its button is not being pressed.    

If all the other data outputs are being used then you will need an external latch which is reset when powered on and set when VT goes high. Something like this:-   







simulate this circuit &ndash; Schematic created using CircuitLab

NOR1 and NOR2 form an SR latch which is reset via NOT1. At power-on C1 is discharged so NOT1's input is low. C1 then charges through R1 producing a high level which terminates the power-on reset pulse.    

When VT goes high it sets the latch and gates NOR3 on. NOR3 then passes the level on D8 through with inversion. All this can be done with a single Quad 2-input NOR IC (eg. 74HC02).