The correct option is B 100o
Given:–––––––––
∙∠A and ∠B form a linear pair.
∙ m∠A=80o
Linear Pair:–––––––––––––––
Pair of adjacent angles whose non-common sides form a straight line and their measures add up to 180o.
∵ ∠A and ∠B form a linear pair.
⇒ m∠A+m∠B=180o
⇒ 80o+m∠B=180o
Subtracting 80o from both sides,
⇒ 80o+m∠B−80o=180o−80o
⇒ m∠B=100o
So, option(b.) is correct.