Which physical property of the alkali metal group decreases down the group

Melting and boiling point

Explanation:

For all group 1 elements, the melting and boiling point of the various members of the group decreases down the group. Hence, as the elements become heavier, they tend to have lower melting and boiling points.

Thus in moving from lithium to francium, there is a steady decrease in the melting and boiling points of elements down the group.

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

answer: e.   sour taste

explanation: acids are those substances which dissociates in water to give ions. as they dissociate to give ions, they act as electrolytes and thus are electrically conductive. acids are sour in taste. example: hcl

bases are those substances which dissociates in water to give ions or accepts ions. as they dissociate to give ions, they act as electrolytes and thus are electrically conductive. bases are bitter in taste. bases are slippery in nature.