How did the british navy protect britain and expand the war around the world?

The United Kingdom's Navy, or Royal Navy, is the tittle used to refer the United Kingdom's naval combat force, whose origin goes to earlies 16th century. This Navy played an important part in establishing the British Empire as well as expanding its supreme world power by contributing in the conquering of overseas possessions and setting of trading posts, gaining the historical reputation of “Royal Navy”.

From the middle 17th century, and 18th century, the Royal Navy fought against the Dutch Navy and the French Navy for protecting Britain maritime supremacy.  During 18th century it was the world's most powerful navy until it was surpassed, during the Second World War, by the United States Navy.

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."