Multiple Choice Option [work .... to earn]

45_ They are such a hard working family and work …………. to earn their living.

a)too hard
b)very hard (Answer Key)
c)too hardly
d)hardly enough

Source: text book

Why not “a” and “c” and “d”?

Thank you

too hard - indicates that they work beyond the point that they need to work. It has a negative meaning in English and cannot be substituted for ‘very hard’ as it does not mean the same thing.
too hardly - hardly means ‘barely’.
hardly enough - is grammatically almost possible but logically, as they have just been described as ‘hard working’ you cannot now say that they barely work hard enough. (that is, they work as little as they can possibly get away with working.)

Thank you.