Using correct verb tenses

To answer questions on verb tenses, you should notice the use of time markers used in the sentence or the tense of the other verb used in the situation related to the action of which verb is missing.
Let’s look at the first example:

1. We___________ for three hours before his plane landed.
    a) waiting
    b) had been waiting
    c) were waited
    d) are waiting
Here you see the time marker “for three hours” which expresses the length of time that the action happened. There are 2 actions: “we wait” and “his plane lands” whereby the former happens and ends before the latter starts. Both actions happened in the past. The action that happened first should be in Past Perfect or Past Perfect Continuous tense (in case the continuity of the action is emphasised) while the action that happened later should be in Past Simple tense. 
The “we wait” action happened and ended before the landing of his plane, so the most appropriate answer is b) had been waiting.
Let’s look at the second example:

2. Generally, people _________ unlabelled products due to safety reasons.
    a) haven’t bought
    b) don’t buy
    c) aren’t bought
    d) buy
You can see the adverb of frequency “Generally” which indicates that the action happens in general. The most appropriate tense should be Present Simple tense as we use this tense for habitual actions or things that happen all the time. Both b) don’t buy and d) buy are Present Simple, but the negative verb makes more sense in this statement. Therefore, the answer is b) don’t buy.