Which one of the following is an irrational number?

Option A is correct since π = 3.141592… i.e. the decimal expansion is non-terminating and non-recurring.

Option B is incorrect because = ±3 and it is an integer (and hence a rational number, Integers ⊂ Rational Numbers).

Option C is incorrect as it is expressed in the form (p = 1 and q = 4) and is a rational number (by definition, q≠0).

Option D is incorrect as it is expressed in the form (p = 1 and q = 5) and is a rational number (by definition, q≠0).

Hence, option A is correct.