This question seems a little bit ambiguous, since we're not told anything about the manner in which a number is changed.

That said let's add 1 to each of the four values, and see what effect the change has on the value of n.

Add 1 to 13 to get: n = (13 + 1)(15)(17)(19)
Expand to get: n = (13)(15)(17)(19) + (1)(15)(17)(19)
In this case, the value of n increases by (1)(15)(17)(19)

Add 1 to 15 to get: n = (13)(15 + 1)(17)(19)
Expand to get: n = (13)(15)(17)(19) + (1)(13)(17)(19)
In this case, the value of n increases by (1)(13)(17)(19)

Add 1 to 17 to get: n = (13)(15)(17 + 1)(19)
Expand to get: n = (13)(15)(17)(19) + (1)(13)(15)(19)
In this case, the value of n increases by (1)(13)(15)(19)

Add 1 to 19 to get: n = (13)(15)(17)(19 + 1)
Expand to get: n = (13)(15)(17)(19) + (1)(13)(15)(17)
In this case, the value of n increases by (1)(13)(15)(17)

Among the four products in red, smallest value is (1)(13)(15)(17), which means adding 1 to 19 will have the least effect on the value of n.

Answer: D