Answer:
C. Also, known as the 3rd one.
Step-by-step explanation:
Answer:
The answer is:

Step-by-step explanation:
Given:
In △ABC:
m∠A=15°
a=10 and
b=11
To find:
c = ?
Solution:
We can use cosine rule here to find the value of third side c.
Formula for cosine rule:

Where
a is the side opposite to 
b is the side opposite to 
c is the side opposite to 
Putting all the values.

The answer is:

The postulate that is used in order to prove the congruency of the triangles is the ASA which means (Angle – Side – Angle). The property that is applicable for the congruency of DB to itself is the reflexive property. Therefore, the answer to this item is the second choice.
Hope I helped! :)