Which of the following statements is/are true?

The correct statement is: For a grammar to be LL(1), it must be left-factored.

Let's analyze each of the statements to understand why this is the correct choice:

1. LL(1) parsers do not use backtracking:
An LL(1) parser is a deterministic, predictive top-down parser. The first 'L' stands for scanning the input from Left to right, the second 'L' stands for producing a Leftmost derivation, and the '1' indicates that it uses exactly one lookahead input symbol to make parsing decisions. Because the parser can uniquely determine which production rule to apply at each step using a parsing table and a single lookahead token, it never needs to backtrack. Thus, the statement "LL(1) parser uses backtracking" is false.

2. LL(1) grammars cannot be left-recursive:
If a grammar contains left-recursion, it has a non-terminal that can eventually derive a sentential form starting with itself, for example:

$$A\to A\alpha |\beta$$

When a top-down parser like LL(1) tries to expand the non-terminal ^A, it can repeatedly apply the rule ^{A → Aα} without consuming any input symbols, causing the parser to fall into an infinite loop. Therefore, left-recursion must be eliminated for a grammar to be LL(1). This makes the statement "For a grammar to be LL(1), it must be left-recursive" false.

3. LL(1) grammars must be left-factored:
Left-factoring is a grammar transformation technique used to remove common prefixes from production rules sharing the same left-hand side. For example, if we have:

$$A\to \alpha {\beta }_1|\alpha {\beta }_2$$

An LL(1) parser looking at the prefix ^α cannot decide which of the two productions to choose based on a single lookahead token. To resolve this choice, we must left-factor the grammar by rewritng it so that the choice is deferred. Consequently, for a grammar to be LL(1), it must be left-factored. This statement is true.

4. SLR parsers are more powerful than LL(1) parsers:
SLR(1) (Simple LR) is a bottom-up parser. Bottom-up parsers generally handle a wider class of grammars than top-down parsers because they defer decisions until they have seen the entire right-hand side of a production. The set of LL(1) grammars is a strict subset of SLR(1) grammars. Therefore, SLR parsers are more powerful than LL(1) parsers, making the statement "The LL(1) parsers are more powerful than the SLR parsers" false.