Answer: (i) Left recursion

Explanation

A grammar is left recursive if it has a rule of the form $A \to A\alpha \mid \beta$.

A top-down (recursive-descent / LL) parser expands the leftmost non-terminal first. For $A \to A\alpha$ it would call A() which immediately calls A() again without consuming any input, so it falls into an infinite loop.

Fix: eliminate left recursion

$$A \to A\alpha \mid \beta \;\;\Longrightarrow\;\; A \to \beta A',\quad A' \to \alpha A' \mid \varepsilon$$

• Right recursion ($A \to \alpha A$) is harmless for top-down parsers.
• Ambiguity affects all parsers, but it is not the characteristic problem of top-down parsing, and option (iii) says "only".

$$\boxed{\text{(i) Left recursion}}$$