For me it would be SEO, in where it can make the content of your web pages more relevant, more attractive, and more easily read by search engines and their crawling and indexing software.

SEO - Search Engine Optimization
SMO - Search Media Optimization
SMM - Social Media Marketing

It really depends what you need, all of them has its own advantages and disadvantages.

SEO -Search engine optimization (SEO) is the process of improving the visibility of a website or a web page in a search engine's "natural" or un-paid ("organic" or "algorithmic") search results.
SMO- SMO (Social Media Optimisation) is the process of preparing a website for use by social media communities. This process includes places social bookmarking buttons on pages, embedding social media tools on pages and developing content specifically targeted at social media communities.
SEM - MM (Social Media Marketing) is the process of entering into social media networks with a strategic plan to enhance your SEO. SMM seeks to increase your visibility within social networking communities by generating links, posts and articles etc with the purpose of driving traffic, increasing popularity and increasing brand awareness.

SEO is free techniques,you select different keywords and getting high ranking in search engine.It also stands for traffic on your website.
SMO- Search media optimization,Using this techniques you getting huge traffic on your website.Use different social media websites like Facebook.
SEM is paid techniques in which you pay for each time someone clicks on your ad.

SEO stands for Search Engine Optimization, SMO stands for Social Media Optimization and
SEM stands for Search Engine Marketing.

SEO & SMO both are good technique to get the best traffic and business.

SEO stands for search engine optimization; SEM is for search engine marketing while SMO means social media optimization. These are the most used tools for website enhancement.

anyone elaborate in brief about the importance as well as the use of these activities for internet marketing

Before read this post i know about SEO. thanks for sharing this

SEO and SMO is best and also easy to do so.

SEO - The term stands for Search Engine Optimization. Optimize your Website to be in SERP's as through Organic Search by following Search Engines SEO Guidelines.
SMO - Social Media Optimization, promoting your Services or Products on Social Media Sites for being more popular about yourself marked as Social Media Optimization.
SEM - Search Engine Marketing worked under Paid Ad's which placed under Google, Bing, MSN as AD's with specific keywords and also known as Non-Organic Search.

You need to use all three of them for best results, as they are interrelated. SEO is impacted by social signals (SMO) and also the volume of traffic that SEM brings in. SEM and SMO in turn are positively impacted if your site has good SEO.

SEO is best for increase website traffic.

Search Engine Marketing is the procedure to enhance the marketing process. Therefore, take the advantage of both SEO and SMO to enhance the visibility of the website.

Organic Process:
SEO-Search Engine Optimization
SMO-Social Media Optimization

Paid Process:
SEM-Search Engine Marketing
SMM-Social Media Marketing

we know all about these.
Thank You