(b) nucleus, chloroplast, vacuole, guard cell
In a closed stoma, (1) nucleus, (2) chloroplast, (3) vacuole, and (4) guard cell are the correct labels respectively.
In a standard diagram of stomata, the guard cells are the outermost bean-shaped structures (label 4), with the nucleus, chloroplast, and vacuole visible inside them. When the stoma is closed, the vacuoles are small (less turgid). Memorise the internal-to-external order: nucleus → chloroplast → vacuole → guard cell. Option (b) follows this standard labelling pattern used in NCERT Life Processes diagrams.