Guard cells of grasses are characteristically:
Correct option is C, Dumbbell-shaped. In STOMATA MOVEMENT problems like this, the accurate statement is Dumbbell-shaped; the others are common mix-ups. Not these: A) Kidney-shaped; B) Circular; D) Rectangular. Tissue transport questions usually turn on xylem vs phloem, living vs dead cells, or source vs sink.
Ref: Best CSIR NET Plant Physiology books: Master Unit 6 with Taiz & Zeiger and Salisbury & Ross. Crack Part C experimental questions with top textbooks.