Explanation

(c)
The octet of all atoms are complete in structures $a$ and$\ b$. The molecule in which all the atoms have completed octet is more stable than atom which have incomplete octet. Larger the number of resonating structures, larger will be the stability, thus structures $a$ and $b$ are stable.
In structure (d), the electron deficient of positive charged carbon is duly compensated by one pair electrons of adjacent oxygen atoms while such neighbour group support is not available in structure (c). Hence, structure (c) is least stable in comparison to structure (d).