A station marked as P.I. on a drawing is referring to?

The choice indicating that P.I. refers to the Point of Intersection of the oncoming and outgoing tangents of a horizontal curve is correct because this terminology is standard in engineering and surveying practices concerning horizontal alignment. The Point of Intersection is a critical point when designing curves in roadways or railways, as it represents where two tangential lines intersect at the curve's center.

In this context, it is vital for understanding how the curvature transitions from a straight path to a curved path and vice versa. This point is essential for calculating the radius of the curve and ensuring safe transitions for vehicles traveling along the path.

The other options do not accurately represent the widely accepted meaning of P.I. in surveying and civil engineering contexts, further reinforcing the correctness of the first answer.