Option B and E eliminated because of the modifier "Occupying a small portion of the island of Borneo" does not points to the "country". Option A simply seems a very bad construction: " international visitors until 2017, when the numbers, attracted by its famously pristine rain forests, increased by 20%"-----> "When the number" is modifying "2017" and "attracted.....rain forest" further modifies "numbers"? too many modifiers pointing to modifiers.
Occupying a small portion of the island of Borneo, [u]the tiny country of Brunei Darussalam had been having a low number of international visitors until 2017, when the numbers, attracted by its famously pristine rain forests, increased by 20%.
A. the tiny country of Brunei Darussalam had been having a low number of
international visitors until 2017, when the numbers, attracted by its famously pristine rain forests, increased by 20%B.
the number of international visitors to the tiny country of Brunei Darussalam was low till 2017 when they began to be attracted by its famously pristine rain forests, leading to an increase in numbers by 20%
C. the tiny country of Brunei Darussalam saw, in 2017, a 20% increase in the previously low number of international visitors because the tourists were attracted to its famously pristine rain forests ---> correct answer
D. the tiny country of Brunei Darussalam increased its number of international visitors by 20% in 2017,
leading to an attraction for its famously pristine rain forests.---> Suggests that increased number resulted in attraction
E.
the international tourists visiting the tiny country of Brunei Darussalam increased their previously low numbers by 20% in 2017, owing to their attraction to its famously pristine rain forests.