An airplane is flying south at 100 km/hr. A wind is blowing west at 25 km/hr. What is the resultant velocity of the airplane?

The airplane's velocity is $100 \text{ km/hr South}$. We can represent this as a vector pointing downwards.
The wind's velocity is $25 \text{ km/hr West}$. We can represent this as a vector pointing to the left.
Since these velocities are perpendicular, we can use the Pythagorean theorem to find the magnitude of the net velocity.

Let $V_p$ be the airplane's speed and $V_w$ be the wind's speed.
The net velocity $V_{net}$ is the hypotenuse of a right-angled triangle formed by $V_p$ and $V_w$.

Substitute the given values:

Rounding to one decimal place, the net velocity is approximately $103.1 \text{ km/hr}$.
Comparing this with the given options:
A. $103.1 \text{ km/hr}$
B. $75.0 \text{ km/hr}$
C. $125 \text{ km/hr}$
D. $26.9 \text{ km/hr}$

The correct option is A.

The final answer is $\boxed{\text{A. } 103.1 \text{ km/hr}}$.