Consider the context-free grammar G below. Here S is the starting non-terminal…

S→aSb∣T

This means:

You can wrap a at the beginning and b at the end repeatedly

Eventually terminate with T=b+
Build the pattern

Let’s apply the rule:

• Apply S→aSb n times

• Then terminate with T=bm, where m≥1

Each time:

• Add one a at front

• Add one b at end

So final string becomes:

an  bm  bn

Which simplifies to: {ambn:0≤m<n}
which is option A.