Step 1: Recall the two kinds of isomerism asked.
Geometrical isomerism comes from different spatial arrangements like cis and trans or fac and mer. Optical isomerism comes from the complex being non superimposable on its mirror image. We want a complex that shows geometrical isomerism only.
Step 2: Examine the diaquadioxalatochromate(III) ion.
$[Cr(H_2O)_2(C_2O_4)_2]^-$ has both cis and trans forms and the cis form is chiral, so it shows both geometrical and optical isomerism. Not our answer.
Step 3: Examine the dichloridobis(ethylenediamine)platinum(IV) ion.
$[Pt(en)_2Cl_2]^{2+}$ also has cis and trans forms, and the cis form is optically active. So it shows both types. Not our answer.
Step 4: Examine the tris(ethylenediamine)cobalt(III) ion.
$[Co(en)_3]^{3+}$ has three identical bidentate ligands, so there is no cis/trans choice; it shows only optical isomerism. Not our answer.
Step 5: Examine triamminetrinitrito-N cobalt(III).
$[Co(NH_3)_3(NO_2)_3]$ is a neutral $MA_3B_3$ complex that exists as facial and meridional forms. Both fac and mer are superimposable on their mirror images, so it shows geometrical isomerism only.
Step 6: Conclude.
The complex with only geometrical isomerism is the triammine trinitrito cobalt(III).
\[ \boxed{\text{Triamminetrinitrito - N cobalt (III)}} \]