Answer: Option 'B' is correct.
Step-by-step explanation:
Since we have given that
ΔABC is an isosceles and ∠B and ∠C are congruent.
So, ∠B=∠C=3x+1
∠A= [tex]40\textdegree[/tex]
As we know the formula for "Sum of three angles in a triangle ":
[tex]\angle A+\angle B+\angle C=180\textdegree\\\\40\texdegree+3x+1+3x+1=180\textdegree\\\\42\textdegree+6x=180\textdegree\\\\6x=180\textdegree-42\textdegree\\\\6x=138\textdegree\\\\x=\frac{138}{6}\\\\x=23\textdegree[/tex]
Hence, Option 'B' is correct.