选C.how to...是一个很常见的结构,意为“怎样能够...”
how to （improve the crop）=how he could （improve the crop）.
这句话的意思是：他苦思冥想怎样能够提高收成.
类似的结构有：what to...=what can/could sb....
I don't know what to do to.=I don't know what I can do.