A) An unambiguous grammar has same leftmost and rightmost derivation

This statement is false, let me take an example, suppose given unambiguous gammer is

S -> AB

A -> a

B -> b

Suppose our input string is ab

Left Most Derivation

S -> AB -> aB -> ab

Right Most Derivation

S -> AB -> Ab -> ab

Notice Left Most derivation and Right most derivation are not same, still gammer is unambiguous.

Here is the Derivation Tree

So for a given gammer, we can have same Left most derivation Tree and Right most derivation tree, but left most derivation and right most derivation may vary.

B) An LL(1) parser is a top-down parser : True

C) LALR is more powerful than SLR : True

D) An ambiguous grammar can never be LR(k) for any k : True

So only False option is (a).