C

Step 1: Understand the System
We have a particle suspended from a string of length \( \ell \) that is given a horizontal speed \( u \). As it moves, the particle will trace out a circular arc due to the tension in the string and gravity acting on it.

Step 2: Speed Calculation
At point B (the lowest point), the particle will have maximum speed, given by energy conservation principles. The potential energy at the highest point (let's say at height h) converts to kinetic energy at the lowest point: \( mgh = \frac{1}{2} mv^2 \). The speed at point B can be computed to be \( v_B = \sqrt{2gh} \), where h is the vertical distance fallen from the initial height. Assuming the initial height is equal to the length of the string, we can say \( h = \ell \). Hence, \( v_B = \sqrt{2g\ell} \). This value corresponds to the speed at B.

Step 3: Speed at Point C
At point C, the particle would be at a height determined by the angle at which it swings. Using trigonometry, one can calculate the height and thus the speed at that point using the same energy conservation principle.

Step 4: Tension Calculations
The tension in the string is affected by both the gravitational force acting on the particle and the centipetal force needed to keep the particle moving in circular motion. At point B, the tension will be at a maximum, given by \( T_B = mg + \frac{mv^2}{r} \), where r is the radius of the circular path, corresponding to the length of the string \( \ell \).

On the other hand, at point C, the tension will be calculated similarly, but taking into account the vertical position of the particle. This can lead to lower tension compared to point B.

Matching:
(i) Speed at B: Matches with [C] (from calculations).
(ii) Speed at C: This will correspond to [A] from calculations, (may depend on specific height or angle).
(iii) Tension at B: [C] corresponds to the maximum tension calculated.
(iv) Tension at C: Matches with [D].

Conclusion:
Thus we have our matching pairs based on the calculations performed.