(a) Tracheal plugs which plug the lumen of the vessels and tracheids
(b) Compound sieve plates
(c) Specialised secretory cells
(d) Laticiferous channels
The balloon-shaped structures tracheal plugs formed in the xylem vessels of heartwood, due to the intrusions of secondary xylem parenchyma are called tyloses.
Hence option a is the answer.