Step 1: Recall the two lightest baryons of the nucleus. The proton and neutron are each built from three first-generation quarks drawn from up ($u$) and down ($d$) types.
Step 2: The proton is $uud$ and the neutron is $udd$. The problem specifies exactly one $u$ and two $d$ quarks, which matches the neutron pattern.
Step 3: Confirm by charge: $u$ contributes $+2/3$ and each $d$ contributes $-1/3$, so the total is $\tfrac{2}{3} - \tfrac{1}{3} - \tfrac{1}{3} = 0$, consistent with the electrically neutral neutron.
Step 4: Electrons and positrons are fundamental leptons and have no quark substructure, so only the neutron is a valid quark triplet here.\[\boxed{\text{Neutron}}\]