Evidence suggests that Isaac Newton, although he published his work on fluxional calculus in the 1680s,
the same decade as Gottfried Leibniz's work on differential and integral calculus were published, had made his discoveries a decade earlier than Leibniz had.
Although is a parallel marker. the Correct form is Although X, Y {X &Y needs to be parallel}.
here X is "he published his work on fluxional calculus in the 1680s". Past tense & Active Voice. Y should also follow the same pattern.
Now lets check the option:
1.the same decade as Gottfried Leibniz's work on differential and integral calculus were published - were is wrong { work is singular} & Passive Voice{not parallel with X}-Wrong
2.the same decade as Gottfried Leibniz's work on differential and integral calculus was published -Passive Voice-Wrong
3.the same decade that Gottfried Leibniz published his work on differential and integral calculus - Active voice and parallel with X, i will keep this.
4.the same decade that Gottfried Leibniz had published his work on differential and integral calculus - HAD published is wrong. no need to use double past. - Wrong
5.the same decade as Gottried Leibniz's published work on differential and integral calculus - AS distort the intended meaning of showing contrast- Wrong.
Correct Ans C