The structure of a main sequence star like our sun can hold for a long time because

A. Gravitation fore is counterbalanced by thermal expansion forces

B. The immense volume of matter

C. The rotational force

B

The immense volume of matter 

Explanation:

Stars are formed by cloud and dust, draw together by gravity.

But the longevity of a main sequence star lives depends on how massive it is. An enormous massive star burns faster due to its enormous material it contains and higher core temperatures caused by greater gravitational forces. While a less massive star will burn slowly and stick around the sun for a very long period. The structure of a main sequence star like our sun can therefore hold for a long time because of the immense volume of matter of star.