Look at the diagram of the Periodic Table - OCR Gateway - GCSE Chemistry: Combined Science - Question 10 - 2020 - Paper 10

To determine which element has the greatest tendency to form positive ions when reacting with dilute hydrochloric acid, we need to analyze the positions of the elements A, B, C, and D in the periodic table.

1. Identify Element Types: Elements that are typically metals tend to lose electrons to form positive ions. Therefore, we should focus on the positions of A, B, C, and D to see which are metals.

2. Trend of Ion Formation: The tendency to form cations increases as you move down a group and decreases across a period. Group 1 elements (alkali metals) are highly reactive and have a strong tendency to lose their outermost electron and form positive ions.

3. Evaluate the Given Elements: Assuming that elements A and C are metals, while B and D are non-metals, element C would be expected to form positive ions more readily than A.

4. Conclude: Therefore, if C is a metal situated in a favorable position indicating high reactivity, it has the greatest tendency to form positive ions when reacting with dilute hydrochloric acid.

Thus, the answer is: C.