Which of these holds a galaxy together? a. disk b. gravity c. open clusters d. globular clusters

'Disk', 'open cluster', and 'globular cluster' are shapes into which stars gather themselves.  

For holding big groups of stars together, we have to find some kind of force. 'Gravity' does the job nicely.  (B)

a

explanation:

because they are cartillidge

one "light year" is the distance that light travels, through vacuum, in one julian year.   that's 365.25 days, of 24 hours each.