Step 1: When a pair of dice is thrown, the total number of equally likely outcomes is
$$6\times 6=36$$
Step 2: A doublet means both dice show the same number: (1,1), (2,2), (3,3), (4,4), (5,5), (6,6) — that is 6 outcomes.
Step 3: Probability of getting a doublet
$$P(\text{doublet})=\frac{6}{36}=\frac{1}{6}$$
Step 4: Use the complement rule.
$$P(\text{not a doublet})=1-\frac{1}{6}=\frac{5}{6}$$
Correct option is (B) $\dfrac{5}{6}$.