Question

Which quantity represents the prefix used in naming straight-chain alkanes with the exception of methane, propane, and butane?

Answer 1

all alkanes end with an -ane ending
as for prefixes
methane = 1 carbon alkane
ethane = 2 carbon alkane
propane = 3 carbon alkane
butane = 4 carbon alkane
pentane = 5 carbon alkane
heptane = 6 carbon alkane
and so on...

Answer 2

just adding to the answer above a good rule of thumb for alkanes is C(n)H(n+2)