Step 1: Link hardness to bonding electrons.
A transition metal is hard when both its $s$ and unpaired $d$ electrons join in metallic bonding, locking the lattice tightly.
Step 2: Spot the soft case.
If the $d$ subshell is completely filled, those $d$ electrons stay paired and tightly held and cannot help bond, so the metal turns soft.
Step 3: Write the configurations.
$Mn$ is $[Ar]3d^5 4s^2$, $Co$ is $[Ar]3d^7 4s^2$, $Ni$ is $[Ar]3d^8 4s^2$, all with unpaired $d$ electrons available for bonding.
Step 4: Examine zinc.
$Zn$ is $[Ar]3d^{10} 4s^2$, with a fully filled $3d$ subshell.
Step 5: Reason out the softness.
Since zinc's $3d$ shell is complete, only its two $4s$ electrons bond. Weak metallic bonding makes zinc soft, malleable and low melting compared with $Mn$, $Co$ and $Ni$.
Step 6: Conclude.
The soft element is zinc, option (B).
\[ \boxed{Zn \text{ (option B)}} \]