Notice that angles G and the third angle (let's call it x) from triangle ABX lay in the same straight line.
This way,
[tex]\begin{gathered} x+G=180\rightarrow x=180-G \\ \Rightarrow x=78 \end{gathered}[/tex]Now, we know that angles A and D add up to 90°. This way,
[tex]\begin{gathered} A+D=90 \\ \rightarrow A=90-D \\ \rightarrow A=60 \end{gathered}[/tex]We also know that the sum of the interior angles of a triangle is 180.
Therefore,
[tex]B+A+x=180[/tex]Solving for B we get:
[tex]\begin{gathered} B=180-A-x \\ \Rightarrow B=42 \end{gathered}[/tex]Angle B measures 42°.
Answer: B. 42°