S→aSb∣SS∣λ
This grammar will generate all strings in which na(w) = nb(w) .
A prefix is a string of any number of leading symbols.
For example, the string abc has prefix (empty string), a, bc, abc. When we consider the prefix v of w, the number of ‘a’s in it is either greater than or equal to ‘b’s in the string w.
For example, aab is prefix of the string aabb. Here, number of ‘a’s is more than the number of ‘b’s. ab is a prefix of the string abab. The number of ‘a’ and ‘b’ are equal. So, when we consider the prefix v, the number of ‘a’s and ‘b’s are equal.
Hence,Option(A) is the Answer.