Step 1 - Recall Werner's terminology.
Primary valency = oxidation number of the metal, satisfied by negative ions, written
outside the coordination sphere.
SECONDARY VALENCY = COORDINATION NUMBER, i.e. the number of ligand donor atoms
directly bonded to the central metal ion. It is non-ionisable and fixed.

Step 2 - Count the ligands inside the square brackets.
$5 \times NH_3$ (each monodentate, donates 1 lone pair through N) = 5
$1 \times NO_2^-$ (nitro, monodentate, donates through N) = 1
Total donor atoms = $5 + 1 = 6$

Step 3 - Conclude.
Secondary valency (coordination number) of Co = 6, and the complex is octahedral.
(For comparison, the primary valency here is $+3$, since $x + 5(0) + (-1) = +2 \Rightarrow x = +3$.)

Answer: (D) 6