Choose the sentence where the underlined word is used correctly.

The correct meaning of the word pedestrian is a person who is walking on the roads. It can also be used to mean something commonplace, uninteresting and ordinary. 

Let's look at the options one by one - 

Option A: A story of spies cannot be an ordinary story - as in, it does not occur in ordinary life. Hence, this is incorrect usage.

Option B: Here pedestrian is used as an antonym of difficult which is wrong usage.

Option C: A highway cannot be a pedestrian one. If a road is meant for only walkers then the right word to use is pathway or walkway.

Option D: This correct usage. It is used as an adjective to indicate a very ordinary lecture.