(a) Proof: 2 Marks
(b) Answer: 1 Mark
(a)Consider the two triangles :

In the two triangles,  
$\angle ABC$ = $\angle PQR$
$\angle BAC$ = $\angle QPR$
$\angle ACB$ = $\angle PRQ$
But clearly, $\Delta ABC$ is not congruent to $\Delta PQR$.
As the sides of triangle are not equal.
Thus, AAA cannot be a congruence condition.
It actually tells that the two triangles are similar, but not congruent.
(b) Since, the three sides of the first triangle is equal to the corresponding three sides of the second triangle, by SSS congruence criterion ΔABC is congruent to ΔDEF.

Steps: 2 Marks
Proof: 2 Marks

In $\Delta ABC$,
OD + OE = DE
Multiplying both sides with 2:
2OD + 2OE = 2DE
DE + 2OE = 2DE (2OD = DE; given in question)
2OE = DE
So, OD = OE -------------- (1)
In $\Delta AOEand\Delta DOC$,
OD = OE [From (1)]
$\angle AOE$ = $\angle DOC$   [Vertically Opposite Angles]
AO = OC   [O is the mid-point of AC]

$\Delta AOE\cong \Delta DOC$   [By SAS condition]
Hence, $\angle EAO$ = $\angle OCD$ = ${70}^o$ [Corresponding parts of corresponding triangles]

(ii) RHS, as in the given two right-angled triangles, one side and the hypotenuse are respectively equal.

In geometry, two figures or objects are congruent if they have the same shape and size.

Definition: 1 Mark
Proof: 1 Mark
If two angles have the same measurement, they are congruent. Also, if two angles are congruent, their measurements are same.
We know that in every right-angled triangle, one angle is ${90}^{\circ }$.
Example: Consider two right-angled triangles $\Delta AJU$ and $\Delta NIV$

In $\Delta AJU,\angle AJU={90}^{\circ }$
and in  $\Delta NIV,\angle NIV={90}^{\circ }$
Since one angle is same in both, we can say that they have one pair of congruent angles.
So, if you take any two right-angled triangles, at least one pair of angles will be congruent, i.e. equal.