The correct answer is A. favourable.
A sea change is a dramatic and fundamental change. In this case, the speaker’s visit to foreign countries caused a dramatic and fundamental change in his outlook and his attitude to the people. This change was positive, as the speaker now has a more favorable view of the world and its people.
The other options are incorrect because they do not accurately reflect the meaning of the sentence. Option B, transformation, is too general. It could refer to any kind of change, positive or negative. Option C, unfavourable, is the opposite of what the sentence is trying to say. The sentence is saying that the speaker’s attitude changed in a positive way, not a negative way. Option D, partial, is also incorrect. The sentence is saying that the speaker’s attitude changed completely, not partially.