IIT-JEE 2007 · Paper 2, Physics question 1

Setting up the resonance tube. The vibrating fork is held over the open end with its prongs in a vertical plane, so that the prongs move up and down and send the sound straight down the air column.

The other options. The first two resonances occur at lengths close to $\lambda/4$ and $3\lambda/4$ (plus the end correction), never at $\lambda$ or $\lambda/2$, so (C) and (D) are wrong.

(A)