From 1994-1999 Nelson Mandela was the president of South Africa

[thegmattutor]

From 1994-1999 Nelson Mandela was the president of South Africa, a time in which the African National Congress were the ruling political party of the country.

(A) From 1994-1999 Nelson Mandela was the president of South Africa, a time in which the African National Congress were the ruling political party of the country.
(B) From 1994-1999 Nelson Mandela was the president of South Africa, when the African National Congress were the political rulers.
(C) From 1994-1999 Nelson Mandela presided over South Africa, while the African National Congress ruled it.
(D) Nelson Mandela was the president of South Africa from 1994-1999, during which the African National Congress was the ruling political party.
(E) Nelson Mandela was the president of South Africa from 1994-1999, however the African National Congress were the ruling political party.

Answer:

SPOILER: D

Source: Original question from blog

[bubai800]

IMO D.
First place Nation congress is singular.
Moreover 'During'' clearly defines the span of time.

[Devil2Death]

The answer D is correct one.
Congress is singular noun & hence should be accompanied by WAS. Hence the answer cannot be A, B or E.
C cannot be the answer since it changes the tone/ intended meaning of original statement.