"Diversified" means different or varied. A "diversified strategy" for advertisement would refer to using lots of different kinds or means of advertising with the hope of reaching many different audiences of potential customers.
Most businesses involved in developing advertising strategy for a product or service start by identifying a target population - the age group, interest group, economic or social class they feel are the most likely consumers for whatever they are hoping to sell. Once that audience is determined, a diversified strategy is developed to coordinate delivering advertising about the product or service in many different ways to provide multiple exposures to the potential audience. The company may purchase advertising time on television channels during the time of shows that their audience is likely to watch; create ads to appear on computers of websites likely to be used; send direct mail advertisements to specific addresses or blanket (send to every address) an area; or purchase billboard advertising along roads or on vehicles such as buses. All these ideas and more are possible ways of exposing potential customers to the product or service being advertised.