what is the resistance of earth(good ground)?

Earth is  a good  conductor, it has less than 5 ohms of resistance.

T Flip Flop used as frequency divider

Similarly To D FF,T FF also used as frequency divider.

D Flip Flop as a Frequency divider how?

D FF called as binary divider or frequency divider by small changes in circuit.
By connecting complement output Q(not Q) to input D,then its output is divided the input with a factor of 2.so it is called as frequency divider.