Output voltage at transistor of given figure
Z1 and Z2 limit output 5 V and Z3 and Z4 will give output 5 V. So
  total output is 10 v. And it will be subcontracted from 15 v so output
  should be wrong.


Zeners don't limit output at 5V because they are not directly connected to the output. Output voltage will be any zener diode voltage (because they are all identical) plus voltage drop across R2 (or R3. They're identical, too).

So, since Q1 is off, Q1 and R4 can be deleted from the circuit. From the remaining circuit, you can easily see that \$V_O = V_Z+ I_{R2}\cdot R2\$, where \$I_{R2}\$ is the current flowing through R2 and \$V_Z\$ is the zener voltage, 5V in your case.

Tip: Zeners are identical, R2 and R3 are 10k and these strings are in parallel. And also R1 is 5k. This might tell you something :)

Now you can find  the answer by yourself.