Aresponsibility of the vice president is to ? declare war if necessary. join the house of representatives if necessary. join the supreme court if necessary. become president if necessary.

Answer D. Become President if necessary.

In case the President of the United States is unable to perform his duty, the Vice-President's most important responsability is to be ready to assume the presidency. There can be many reasons for this: Impeachment,  President's death, resignation or temporary incapacitation. He also serves as President of the United States Senade.

for this case we have the following equation:

we apply distributive property to the terms within parentheses.

subtracting 2 on both sides of the equation:

the answer would be c