Different nations see colours differently. What is red to Indians looks almost orange to most other nations. The English are not sure about the border … between violet and purple.

(a) circle
(b) square
© line
(d) dot

Which word fits here?

Which word do you think makes most sense in connection with a “border”.

“line”, I think…

Am I right? Please confirm my option…

Yes, correct.

(It is arguable whether “border line” should be two words or one in this case.)

Thanks, I 've got it :-).