Either oftener or more often. Since often is a native English adjective, not an import, its native comparative is oftener. The form "more often," derived from French grammar, may in some circumstances "sound better" but it is not more correct. Oftener was frequently used by popular late 19th century and early 20th century writers such as Lucy Maud Montgomery and Wilkie Collins, but is far less common in spoken or written English now.
The comparative form of solid is solider
The comparative form of in is inner.
comparative - merrier superlative - merriest
"Shine" is a verb and a noun and, as such, does not have a comparative form. The comparative form of the adjective shiny is shinier.
The comparative form of brown is "browner". The superlative form is "brownest".
The comparative form of much is 'more'. The superlative form of much is 'most'.
more
more
much,more,most
The comparative form of busy is busier.
The comparative form of friendly is friendlier.
The comparative form of dreamy is dreamier
more, most
more, most
comparative form is cleaner superlative form is cleanest
The comparative form of many is more
The comparative form of "central" is "more central."