Many a boy ___ playing football at that time.A.is B.was C.are D.were

正确：B“many a＋单数可数名词”形式上为单数,但意义上为复数(意为“许多”).
若用作主语,其谓语通常与形式一致而不与意义一致(即谓语用单数)
给个例句：
Many a successful store has paid its rent cheerfully.
许多成功的店铺都非常乐意地付了房租