In this question, the work rate of A and B is given as 1 task/10 days and the work rate of B is given as 1 task/15 days. Now, we know that rate = work/time. Hence, the rate of A and B together is 1/10 and rate of B is 1/15. Now, rate of A = rate of A and B together - rate of B = 1/10 - 1/15 = 1/30. Hence, the time taken by A to complete the work alone is 30 days.

Option 1 suggests A can complete the work in 20 days, which is inaccurate since A is slower than B.

Option 2 suggests A can complete the work in 24 days, again this is inaccurate as A`s work rate is slower than that.

Option 3 suggests A completes the work in 25 days, however, this is inaccurate as well.

Option 4 is the correct answer, as it correctly calculates the time A needs to complete the work when working alone, based on the given rates of work.