The correct option is D traveling by ship
The comparison is between 'traveling by plane' and 'traveling by ship', whereas the underlined phrase is 'to travel by ship.' 'to travel' is not in the same form as 'traveling'. This is an error, hence, we need to correct it. Option A is, thus, incorrect. The construction of the phrases which are being compared need to be the same, so we use option D, which has the same structure in 'traveling by ship' as in the phrase 'Traveling...by plane'. Options B and C have demonstratives 'that' and 'those' which are used to modify nouns. Since these options modify the gerund 'traveling' (verb form ending in '-ing'), and they don't modify nouns, they're incorrect.