Why does Trans-Saharan trade fall out of style?

Explanation:

El comercio transahariano se refiere al tráfico de mercancías a través del Sahara hasta alcanzar África Subsahariana desde la costa del Norte de África, Europa o el Levante. Si bien ha existido desde tiempos prehistóricos, el apogeo de esta ruta comercial se produjo entre los siglos VIII hasta el XVI.

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."