Fill in the blank with the most appropriate word.
When I came into the room she stopped _______ T.V.
A) watching
B) watch
C) watched
D) on watching
E) watches

Hint: Tense means the time at which the action of the verb takes place. Present, Past and Future are the three different main tenses. In English, the three principle tenses can take four primary angles: simple, perfect, continuous (also known as progressive), and perfect continuous.

Complete answer:
 In the given sentence, the word that should be used in the blank must act as a noun, thus, becoming the subject of the sentence and also like a verb taking up the object. In English, a gerund has the properties of both verb and noun, like being modifiable by an adverb and being able to take a direct object.

Let us analyze the given options:
Option A) watching: The above option is correct as ‘watching’ is a gerund which is formed with verbs using ‘-ing’ and acts as nouns. Whether you should use a gerund or an infinitive depends on the principal or main verb in the sentence. A gerund refers to the action of the verb nominally, so it can be used as a subject or object. Not every word ending with -ing is a gerund. Some may be participles, functioning as verbs, or even function as gradable adjectives.
Option B) watch: The above option is incorrect as ‘watch’ is in the present tense and doesn’t act as a noun.
Option C) watched: The above option is incorrect as ‘watched’ is in the past tense and doesn’t act as a noun.
Option D) on watching: The above option is incorrect as ‘on watching’ is in the continuous tense and doesn’t act as a noun.
Option E) watches: The above option is incorrect as ‘watches’ is in the present perfect tense and doesn’t act as a noun.

Thus the correct answer is option ‘A’.

Note: Tenses structure the foundation of the English language. The strained, which is generally fitting to communicate the thought, ought to be utilized; else the assertion/question would communicate something inverse to what you needed to state or compose. Right utilization of tense will infer the utilization of the right type of action word with appropriate helper.