Which built-in formula function can find the lowest number from a range of cells?

You can use the MIN function. If your range was A1 to A15, then your formula would be:

=MIN(A1:A15)

You can also use the SMALL function, although it is more for when you want to find a position like the second smallest or third smallest. To use it to find the smallest you would type:

=SMALL(A1:A15,1)

The 1 tells it to find the smallest. 2 would mean the second smallest and so on. To find the smallest it is better and simpler to use MIN.