The correct option is B forming
In the given context, option B best befits the context. The poet, here, is addressing his own soul ('thou' refers to the soul) and it is this soul of his that cautions him against the evil thoughts and actions that were forming within and in the outside world in the last stanza of the poem. In this context, 'brewing' must imply forming. We can eliminate the other options as they simply do not fit the context - they distort the meaning of the poem. Therefore, option B - forming- is our answe