Which number is an irrational number?

A. 0.12
B. 0.1257486. . .
C. 0.12121212. . .
D. 0.111111. . .

The answer is B. 0.1257486... An irrational number cannot be expressed as a finite decimal or a repeating decimal pattern. Option B represents a non-repeating, non-terminating decimal, which is an irrational number.