Explanation:
To determine which pair is the most powerful, we need to understand the concepts of SLR, LALR, and Canonical LR parsers.

SLR (Simple LR) Parser:
The SLR parser is a bottom-up parser that uses a simplified version of the LR(0) parsing algorithm. It has a simple parsing table that contains shift and reduce actions. However, it is not as powerful as other parsing algorithms and may produce shift/reduce or reduce/reduce conflicts in certain grammars.

LALR (Look-Ahead LR) Parser:
The LALR parser is also a bottom-up parser that uses a more powerful version of the LR(0) parsing algorithm. It improves upon the SLR parser by merging states with similar LR(0) items and resolving conflicts by using look-ahead symbols. This reduces the size of the parsing table and avoids some conflicts present in the SLR parser.

Canonical LR Parser:
The Canonical LR parser is a more powerful bottom-up parser that uses the full LR(0) parsing algorithm. It is able to handle a larger class of grammars than both the SLR and LALR parsers. It constructs a set of LR(0) items for each state and builds a parsing table based on these items. This parser is more complex and may have a larger parsing table compared to the other parsers.

Comparing the Pairs:
Now let's compare the given pairs to determine the most powerful one.

a) SLR, LALR:
- SLR is less powerful than LALR.
- LALR improves upon SLR by merging states and resolving conflicts.
- Therefore, LALR is more powerful than SLR.

b) Canonical LR, LALR:
- Canonical LR is more powerful than LALR.
- Canonical LR uses the full LR(0) parsing algorithm.
- LALR is a simplified version of the LR(0) parsing algorithm.
- Therefore, Canonical LR is more powerful than LALR.

c) SLR, Canonical LR:
- Canonical LR is more powerful than SLR.
- SLR is a simplified version of the LR(0) parsing algorithm.
- Therefore, Canonical LR is more powerful than SLR.

d) LALR, Canonical LR:
- Canonical LR is more powerful than LALR.
- LALR is a simplified version of the LR(0) parsing algorithm.
- Therefore, Canonical LR is more powerful than LALR.

Conclusion:
From the above comparisons, we can conclude that the most powerful pair is SLR, Canonical LR. Canonical LR is the most powerful among the given parsers, and SLR is the least powerful.