The correct option is
D help
The given sentence expresses the request of the speaker to the listener, ie. the intention of the speaker to help the listener as the bag is heavy.
It expresses imperative mood.
Imperative mood is used to express a command, request or prayer.
Imperative is formed by using the verb in bare infinitive form.
The imperative mood can only be used in the second person because the person commanded must be the listener. But 'Let' can be used with similar idea with the first and third person, as in the given sentence.
Therefore, the appropriate option to complete the sentence is -
Option A- help
The completed sentence, thus, becomes: That bag is too heavy for you. Let me help you.
Option B is ruled out because it expresses infinitive form of verb. Infinitive form of verb is used to express the purpose or the opinion of something which is not the case in the given sentence.
Option C is also incorrect because it shows verb with 'ing' ie. a gerund or a participle which again express the non-finite form of verb, and such forms are used to express the purpose of something.
Option D is also not suitable to complete the given sentence because it indicates use of simple past tense. Simple past tense indicates the action which has happened / occurred in the past.