Since ab=1 and 1=bc, bc=ab by which property?
is it substitution or transitive and why?

Substitution.

Here is an example.

Let x be equal to 3 and y equal to 3.

From this we can conclude that the values of both x and y are equal to three therefore x and y have the same value and are equal.

Here in your case we have:

Hope this helps.

r3t40