a and c
for infinitely many solutions we require both left and right sides to be equal
distribute and simplify
a
14 - 8x = 20 - 8x - 6 = 14 - 8x ← both sides equal so infinite solutions
b
6x - 54 = 6x - 54 + x = 7x - 54 ← not equal
c
4x + 16 = 4x + 10 + 6 = 4x + 16 ← both sides equal so infinite solutions
d
5x - 40 + 10 = 5x + 10
5x - 30 = 5x + 10 ← not equal