When the length of a pendulum is quadrupled, its time of swing increases proportionally. The relationship between the length of a pendulum (L) and its time period (T) is described by the formula T = 2π√(L/g), where g is the acceleration due to gravity. If the length (L) is quadrupled, it means L becomes 4L. Substituting 4L into the formula gives T = 2π√(4L/g), which simplifies to T = 2π√(4/g)√L. √(4/g) is a constant, so it comes out of the square root, yielding T = 2π(2/√g)√L. Thus, the time period becomes four times the original value. Therefore, the correct answer is option [D]: becomes four times. This illustrates the direct relationship between the length of a pendulum and its time period.