Begin with antibody class and target. Anti-Ro ($SS\text{-}A$) and anti-La ($SS\text{-}B$) are maternal $IgG$ autoantibodies, classically of Sjögren syndrome and lupus. Their $IgG$ nature lets them traverse the placenta into the fetal circulation.
Once in the fetus they bind antigens expressed in fetal tissues, including the cardiac conduction system. The result is neonatal lupus erythematosus - a passively acquired, self-limited skin eruption that fades as the borrowed maternal antibody is catabolised, accompanied by the feared and frequently irreversible complication of congenital complete heart block from inflammatory injury and fibrosis of the atrioventricular node. Affected infants may also show transient cytopenias and transaminitis.
The other options arise from entirely different maternal antibodies: anti-$AChR$ (neonatal myasthenia), anti-$Rh(D)$/blood-group (haemolytic disease of the newborn) and stimulating anti-$TSH$-receptor antibodies (neonatal Graves).
\[\boxed{\text{Maternal anti-Ro/anti-La} \Rightarrow \text{Neonatal lupus} \pm \text{congenital heart block}}\]