Sliding filament theory can be best explained as

• When myofilaments slide pass each other actin filaments shorten while mysoin filament do not shorten

• Actin and Myosin filaments shorten and slide pass each other

• Actin and Myosin filaments do not shorten but rather slide pass each other

• When myofilaments slide pass each other, Myosin filaments shorten while Actin filaments do not shorten

C.

Actin and Myosin filaments do not shorten but rather slide pass each other

Sliding filament theory is explained as actin and myosin filaments do not shorten but rather slide pass each other. 
Two groups of workers, i.e. Andrew Huxley and Ralph Niedegerke (1954) and Hugh Huxley and Iean Hanson (1954) proposed the sliding filament theory. It has essential feature as follows:
(i) During musocle contraction the thin myofilaments show sliding inward towards H-zone. 
(ii) The sarcomere shortens, without changing the length of thin and thick myofilaments. 
(iii) The cross bridge of thick myofilaments connect with portions of thin myofilaments. These cross bridges move on the surface of the thin and thick myofilaments over each other.
(iv) The lengths of thick and thin myofilaments don't change thin filament (actin) during muscle contraction.