The main purpose of audio-visual aids in language teaching is –

Concept

Audio-visual aids are teaching materials that combine the auditory and visual channels together – for example, sound-synced videos, or spoken narration paired with charts, pictures, or models – so a learner receives the same content through more than one sense at once. Educational theory (Edgar Dale's Cone of Experience) holds that learning and retention improve as instruction moves from purely verbal/abstract input toward concrete, multi-sensory experience.

Application

In language teaching, this is exactly why audio-visual aids are used: to activate the students' various senses together, so that words, sounds, and visuals reinforce each other and comprehension of the language content improves.

Cross-check

Measured against this same sensory/comprehension rationale, the other options describe outcomes that lie outside it:

• Simplifying the teacher's work is a workload effect on the teacher, not a change in how students perceive or process the language content.

• Informing students about new equipment makes the equipment itself the topic of instruction, rather than a channel through which the language content is perceived.

• Keeping the class controlled is a discipline/management outcome, not a sensory or comprehension effect on the students.

So the description that matches how audio-visual aids function pedagogically is that they activate the students' various senses together.