Elements are classified into s, p, d, and f blocks according to the subshell into which the last electron enters and the subshell being progressively filled. The configuration ending in 3d^5 4s^1 corresponds to chromium, where the differentiating electrons occupy the 3d subshell, so the element belongs to the d-block of transition elements. The half-filled 3d^5 and singly occupied 4s arrangement reflects the extra stability of the half-filled d subshell. The s-block is wrong because its elements end in ns^1 or ns^2 with no partly filled d subshell. The p-block involves filling of np orbitals. The f-block involves filling of inner 4f or 5f orbitals. Identifying block from electronic configuration is a basic NCERT classification skill. Carefully relating the data to the governing principle ensures the reasoning remains valid even when the numbers or species in the question are changed. Plausibility check: chromium is a well-known transition metal with variable oxidation states and coloured ions, hallmarks of the d-block, confirming the classification.