Saltus is a mathematical term that refers to a discontinuity or jump in a function. It occurs when there is a sudden change in the value of the function at a specific point. In this case, we are considering a saltus at a point of continuity, which means that the function is continuous everywhere except at that particular point.

Saltus at Point of Continuity
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Saltus at a point of continuity can be defined as a jump or a sudden change in the value of a function at a point where the function is otherwise continuous. In other words, the function does not have any breaks or holes in its graph except at the point of saltus.

Options
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a) Positive Number: This option suggests that the saltus at a point of continuity is a positive number. However, this is not always the case. The saltus can be positive, negative, or even zero.

b) Negative Number: This option suggests that the saltus at a point of continuity is a negative number. Similar to the previous option, the saltus can be positive, negative, or zero. So, this option is also incorrect.

c) Zero: This option correctly states that the saltus at a point of continuity is zero. When the saltus is zero, it means that the function has a jump, but the magnitude of the jump is not significant enough to be considered positive or negative. The function simply maintains the same value at the point of saltus.

d) None of these: This option suggests that none of the given options are correct. However, the correct answer is option 'C' as explained above.

Conclusion
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The saltus at a point of continuity is zero. This means that there is a jump in the function at that point, but the magnitude of the jump is not significant enough to be considered positive or negative. It is important to note that saltus can also occur at points of discontinuity, where the function is not continuous. In such cases, the saltus can be either positive or negative, depending on the direction of the jump in the function.