If D is the depth of water upstream of the throat above its sill, B is the width of the throat, to achieve critical flow in an open venturi flume, the theoretical maximum flow Q, is A. Q = 1.71 $${\text{B}}{{\text{D}}^{\frac{1}{2}}}$$ B. Q = 1.71 BD C. Q = 1.71 $${\text{B}}{{\text{D}}^{\frac{3}{2}}}$$ D. Q = 1.71 $${\text{B}}{{\text{D}}^{\frac{2}{3}}}$$

The correct answer is: A. Q = 1.71 $${\text{B}}{{\text{D}}^{\frac{1}{2}}}$$

The theoretical maximum flow Q in an open venturi flume is given by the following equation:

$$Q = 1.71 \sqrt{\frac{B}{D}}$$

where B is the width of the throat and D is the depth of water upstream of the throat above its sill.

Option B is incorrect because it does not include the square root of D. Option C is incorrect because it does not include the factor of 1.71. Option D is incorrect because it does not include the square root of B.