Answer:
Option (A).
Explanation:
From the figure attached,
ΔMON is a right triangle and coordinates of the points M and N are M(0, 2b) and N(2a, 0).
Coordinates of midpoint P →

→ (a, b)
From the formula of the distance between two points,
d =

MN =

=

= 2

Similarly, OP =

=

Therefore, OP =

and MN =
= 2

Option (a) is the answer.