A

Let's analyze each option in detail:

(i) Constant positive acceleration:
When an object has a constant positive acceleration, its velocity will continuously increase over time. Therefore, the speed of the object must increase, making this option match with [B] Speed must increase.

(ii) Constant negative acceleration:
In the case of constant negative acceleration (also known as deceleration), the speed of the object will decrease over time. Thus, this option matches with [A] Speed may decrease.

(iii) Angle between acceleration and velocity is 90º:
When the angle between acceleration and velocity is 90º, it indicates that the object is changing direction without changing its speed. This situation would result in a constant speed, which means this option matches with [C] Speed is constant.

(iv) Angle between acceleration and velocity is less than 90º:
If the angle is less than 90º, it indicates that the object is speeding up because the acceleration is in the same general direction as the velocity. Hence, this situation leads to an increase in speed, which cannot match with any of the given options.

Therefore, the correct matching is:
(i) [B], (ii) [A], (iii) [C], (iv) [D]. Hence the answer primarily aligns when focusing on [A] for (ii). Thus the answer is A.