What elective subject to choose for electronics and communication engineering?

Among the four you have to select one as an elective subject. All the four given are good subjects and all are important only. You have mentioned that you are good at C **. If you select that subject as an elective subject it may be an easy task for you to complete that course. But you are not learning a new skill.
These days Python application programming is getting more important. I know students who are studying B.Tech CSE also that are going for additional certification in Python language. They say that their employment chances will become better if they learn Python also. So I feel you can select that subject as an elective.
But before taking a decision you can also look into the syllabus and other aspects and see how much it will suit you and then can make a decision.

For electing a course from the given options you might have to find out your interest area because then only chances of excelling in that would be more concrete. If you have an interest in coding then definitely Python would be a good choice because nowadays many companies and organisations dealing with the IT projects are switching to Python due to its versatile and structured approach to coding. An expertise in Python would help in securing a job also in the industry.
If you have interest in the fundamental structure of the computer architecture and software to run it then it would be a good thing to go for specialisation in operating system and then learn as how to amalgamate various applications and utilities to it. This area has also good potential and such experts are needed in the IT industry. Professionals who understand and can negotiate with the operating systems are very important as any new project would seek their contribution to link it to the basic framework.
For getting a job what is required is good proficiency in an area and excelling in any area would give same opportunity. There is a lot of competition in the market and industry but for those who have expertise and good understanding of the subject in their chosen domain, there is no problem in getting a job.

Of the four subjects indicated above are quite good in terms of your career prospects giving you an additional edge. However, while choosing anyone as an elective subject, make an intelligent analysis of your choice and passion. You need not think that Python Language would be helpful to you since majority of the aspirants are opting for this programming language. Of course, this would offer you versatility in the coding domain.
Due to its wide application in the areas of Graphic Design, Database Aces, Business Application, Prototyping etc, you will accrue a lot of benefits with the exposure of this language. Even a certificate course in the Python language could be helpful to many aspirants pursuing other streams of engineering in order to secure a prospective job in this area.
Operating system can be selected if you are interested to know the various tools of applications to get the desired outputs in the programme.
However, you should not be in a hurry to chose any subject since the demand in a particular subject has picked up recently. It should be your well thought planning while choosing your domain.