Which of the following is not a sentence?

A. Winding up the pole

B. The ferret has a silly look on its face

C. I really do like spinach

D. The men's bathroom is out of order

A sentence refers to the pair of words which is complete in itself. In other words, a sentence contains a subject and a predicate or it may contain a main clause and one or more subordinate clauses to convey a command, exclamation, question or statement.

In the following statements, the sentences have subject and predicate,

B. The ferret has a silly look on its face.

'The ferret' - subject

'has a silly look on its face' - predicate

C. I really do like spinach.

'I' - subject

'really do like spinach' - predicate

D. The men's bathroom is out of order.

'The men's bathroom' - subject

'is out of order' - predicate

Hence, the sentence 'Winding up the pole' doesn't have a subject that's why it is not a sentence.