What Is the Correct Form of the Equilibrium Constant for This

What is the correct form of the equilibrium constant for this reaction? 2H?O?(g) ? 2 H?O (g) + O?(g)

A) $\frac { \left[ \mathrm { H } _ { 2 } \bigcirc \right] } { \left[ \mathrm { H } _ { 2 } \bigcirc \frac { 1 } { 2 } \right] }$
B) $\frac { \left[ \mathrm { O } _ { 2 } \right] \left[ \mathrm { H } _ { 2 } \mathrm { O } \right] ^ { 2 } } { \left[ \mathrm { H } _ { 2 } \mathrm { O } _ { 2 } \right] ^ { 2 } }$
C) $\frac { \left[ \mathrm { H } _ { 2 } \bigcirc \right] ^ { 2 } } { \left[ \mathrm { H } _ { 2 } \bigcirc 2 \right] }$
D) $\frac { \left[ \mathrm { H } _ { 2 } \bigcirc \right] \left[ \mathrm { O } _ { 2 } \right] } { \left[ \mathrm { H } _ { 2 } \mathrm { O } _ { 2 } \right] }$