The classification of Micro, Small, and Medium Enterprises (MSMEs) in India is based primarily on two factors: Annual turnover and Investment in plant and machinery or equipment. Let's explore each option provided to determine which ones are correct.

1. Annual turnover: As per the MSME Development Act, the revised classification includes annual turnover as a key criterion. This factor reflects the revenue capacity of the enterprise and is crucial for MSME categorization.
2. Number of workers: This factor is not used in the official classification of MSMEs. While the number of workers may impact the operational scale of a business, it is not a statutory measure for classification in India.
3. Investment in plant and machinery or equipment: This is a traditional criterion used for classification under the MSME Development Act. It indicates the scale of technological and capital investment within the enterprise.

Based on the above analysis, the correct factors for classifying MSMEs are Annual turnover and Investment in plant and machinery or equipment. Thus, the correct answer is:

1 and 3 only

This classification helps in determining the eligibility for various benefits, subsidies, and schemes provided by the government to nurture and promote MSMEs in India. By understanding these criteria, stakeholders can better align their business practices with governmental policies and take advantage of the available resources.