If there's no air resistance, then the diver keeps moving horizontally at 40 m/s, even after s/he jumps out of a perfectly good airplane.
Also, gravity ADDS 9.8 m/s of downward speed for every second the diver falls.
One second after leaving the airplane, the diver is moving horizontally at 40 m/s, and moving vertically downward at 9.8 m/s .
The question asks for the "downward velocity". That's 9.8 m/s .
The diver's TOTAL velocity is
√ (40² + 9.8²)
= √ (1,600 m²/s² + 96.04 m²/s²)
= √ (1,696.04 m²/s²)
= 41.2 m/s in some direction between
straight forward and straight down.
