Explanation

Now let's evaluate the options:

(A) ab: This string matches $ab^*$ (since $b^*$ allows for one 'b'). So, this belongs to $\mathrm{R<b\; style="color:\; rgb(0,\; 0,\; 0);\; font-family:\; Overpass,\; sans-serif;\; font-size:\; 0.875rem;"><br></b>}$

(B) abb: This string also matches ab∗ (since $b^*$ allows for two 'b's). So, this belongs to $R$.

(C) cb: This string matches cb∗ (since $b^*$ allows for one 'b'). So, this belongs to $R$.

(D) All of these: Since all the previous options (A, B, C) belong to $R$, this is correct.