Which rational number is one tick mark to the left of - 3/4

I'm assuming each tickmark is 1/4 = 0.25

If that assumption is correct, then going one tickmark to the left of - 3/4 lands you on - 4/4 which simplifies to - 1.

So - 1 is one tickmark to the left of - 3/4

Again, this all hinges on the assumption that each tickmark is 1/4 a unit away from its neighbor.