Step 1: Plan the answer.
First we define the ligand and name one example.
Then we use the shape of the complex to name its isomerism.
Step 2: Part (a) - meaning of didentate.
The word didentate means two teeth.
The ligand grips the central atom with two donor atoms at the same time.
Because of this double grip, a closed ring (chelate ring) forms.
Step 3: Part (a) - a common example.
Oxalate ion is a good example.
\[ \mathrm{{}^-O{-}C(=O){-}C(=O){-}O^-} \]
Both negative oxygen atoms bind to the metal, giving a five-membered ring.
Ethane-1,2-diamine also works as an example.
Step 4: Part (b) - oxidation state and shape.
Let the oxidation state of Pt be $x$.
\[ x + 0 + 0 + 2(-1) = 0 \quad\Rightarrow\quad x = +2 \]
Pt(II) is a $\mathrm{d^8}$ ion and gives a square planar shape with four ligands.
Step 5: Part (b) - count the arrangements.
Four positions lie at the corners of a square.
Place the two Cl ligands on adjacent corners and we get the cis form.
Place them on opposite corners and we get the trans form.
These forms differ only in the spatial arrangement, so the isomerism is geometrical.
Final Answer:
(a) A ligand with two donor atoms, for example oxalate ion or ethane-1,2-diamine.
(b) Geometrical (cis-trans) isomerism.