The bacteriophage attaches itself by its tail to the cell wall of bacterium at:

Explanation

Bacteriophage attaches on specific receptor site on the surface of bacterial cell.The bacteriophage replicates only inside the bacterial cell. The first step in the replication of a bacteriophage is its attachment (adsorption) to host cell at receptor site on the cell wall of bacterium. During attachment, week chemical union between virion and receptor site takes place.So, it is the correct answer.