Answer:
Option A.
Step-by-step explanation:
It is given that a quadrilateral has congruent side lengths. It means the quadrilateral is either square of rhombus.
Each interior angles of a square is a right angle.
2 opposite angles have measures of 100 degrees. The other 2 opposite angles have measures of 70 degrees and (5 x) degrees.
Since the interior angles of the given quadrilateral are not right angles, therefore it must be a rhombus.
Opposite angles of a rhombus are congruent.

Divide both sides by 5.


The value of x is 14.
Therefore, the correct option is A.