The correct option is
C than
Answer : C
Explanation:
"More" in the sentence (question) suggests us that the crowd is comparatively greater (in terms of number of people) is there in London Trains than those in Paris.
Therefore we have to use a word which conveys the same and completes the sentence.
The most suitable word is "than".
The words "like" and "as" can be used in sentences which suggests that the crowd (in terms of number of people) is comparable in both the scenarios.
This excludes the options B and D.
The word "that", does not grammatically fit in the sentence. Which excludes the option A.
Thus, we are left with option C, which is the word that fits in just right.
Thus the complete correct sentence here is:
Trains in London are more crowded THAN in Paris.