Which of the following is most likely to shorten incandescent lamp life?

Operating over rated voltage is indeed the factor that is most likely to shorten incandescent lamp life. When an incandescent lamp is powered above its specified voltage rating, it causes the filament inside the bulb to heat up excessively. This increased temperature leads to a more rapid deterioration of the filament material. As the filament wears down due to the higher thermal stress, the chances of the filament breaking increase significantly, resulting in a reduced operational lifespan for the lamp.

In contrast, operating under rated voltage generally allows the lamp to work cooler, which can actually prolong its life. Similarly, operating the lamp base up is usually a standard position and does not adversely affect the lifespan. Lastly, while high ambient temperatures can influence lamp life, an ambient temperature of 35°C is within a range that most incandescent lamps can handle without being severely affected. Thus, the correct reasoning supports that operating over rated voltage is detrimental to the longevity of incandescent lamps.