Which sentence is a strong research question? Why did the American Revolution occur?
How long did the American Revolution last?
How many people died in the American Revolution?
Who were the leading politicians during the American Revolution?

ill need more info in order to answer yer ?

the present continuous is a verb tense which is mainly used to indicates that an action is in progress at the moment of speaking.  

it is formed using: person + verb to be (am/is/are) +verb + - ing form  

example: i am studying at this moment .

in addition, the present continuous is also used in order to express things that will or will not happen in the near future.

example: i am having dinner with some friends tonight.

the speaker sometimes   uses it   to express that an action is repeated and it causes irritation on the part of the speaker

example: my sister is constantly complaining about everything.

i cannot see any of the options

i would have to say "a" or "d".