What is a cross-bridge?
a. a protein found along the groove of the F-actin double helix
b. a T tubule and two adjacent terminal cisternae
c. the combination of myosin heads with active sites on actin molecules
d. the movement of myosin head while attached to actin myofilament
e. after exercise, the oxygen taken in that exceeds the oxygen required for resting metabolism

The correct answer is - option C.

Explanation:

Cross bridge is a structure that is globular made up of the actin filaments and activated myosin attachments. The head of the myosin binds to the binding site that is present on the actin filaments that are exposed to the actin molecules.

Due to this structure, the movement in muscle occurs as they contract while attaching temporarily to the actin filament that is present near and exposed.

Thus, the correct answer is - option C.

the last one: artificial selection/genetic engineering

plant breeding is modifying the genetic composition of the plant - but this isn't cloning (which is specifically introducing a gene into a genome). but you are artificially selecting for the phenotype that you want.

the second one is literally genetic engineering - the gene would have been cloned first and then inserted or genetically engineered into the new plant cell (the cotton).