C

Step 1: Calculate the horizontal distance covered by the object while falling.
Since the plane is flying horizontally at a speed of 98 m/s and the object is in free fall for 10 seconds, the horizontal distance (d) covered is given by:
d = speed \times time
d = 98 \, \text{m/s} \times 10 \, \text{s} = 980 \, \text{m}

Step 2: Calculate the vertical distance fallen by the object.
Using the formula for vertical distance in free fall:
h = \frac{1}{2} g t^2 where g = 9.8 \, \text{m/s}^2 (acceleration due to gravity), we get:
h = \frac{1}{2} \times 9.8 \, \text{m/s}^2 \times (10 \, \text{s})^2 = 490 \, \text{m}

Step 3: Determine the angle of impact.
Using the tangent of the angle (\theta) formed with the horizontal:
\tan(\theta) = \frac{\text{opposite}}{\text{adjacent}} = \frac{h}{d}
\tan(\theta) = \frac{490}{980} = 0.5

Step 4: Calculate the angle.
Using the arctangent function:
\theta = \tan^{-1}(0.5) = 26.57º. However, to find the angle of impact made by the object while hitting the ground with respect to the horizontal plane, we need to consider the supplement of this angle:
Angle of impact = 90º - 26.57º = 63.43º and rounding it we have approximately 60º.

Therefore, the correct option is C: 60º.