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Weak bases favor E1 reactions, while strong and bulky bases favor E2 reactions.So the weak bases like water and methanol favor E1 reactions while strong bases like ethoxide favor E2 reactions and bulky bases like tert-butoxide too favor E2 reactions.

The tertiary halide will be the most reactive to undergo elimination reaction followed by secondary and primary halide.

Primary halides undergo E2 reactions, while secondary and tertiary halides can undergo both E1 and E2 reactions depending on the nature of the base used.

Keeping into account all the key points, the products with their mode of reaction are shown below:-

Formation of products in reaction a

E2 is favorable here.

Formation of products in reaction b

E1 is favorable here

Formation of products in reaction c

E1 is favorable here.

Formation of products in reaction d

E2 is favorable here.