The cross bridge cycle starts when

a) ca2+ is actively transported into the sarcoplasmic reticulum
b) ca2+ from the sarcoplasmic reticulum binds to troponin
c) atp binds to troponin and is hydrolyzed to adp
d) pi ca2+ from the sarcoplasmic reticulum binds to tropomyosin
e) acetylcholine diffuses away from the synaptic cleft

b) Ca2+ from the sarcoplasmic reticulum binds to troponin

Explanation:

The release of the calcium ions initiates the muscle contractions. The release is triggered from the storage in sarcoplasmic reticulum. These ions initiates contraction that is sustained by the ATP .The ions from sarcoplasm bind to the troponin which drive cross-bridge cycling and pulling of the actin strands by the myosin.