The correct options are
B Up
C Upon
The verb, 'climb' is usually used along with two prepositions viz. 'up' and 'down' which indicate the direction of climbing. Eg: 'Climb up a tree' and 'Climb down a tree'. In the given sentence, the direction of climbing is up. This is because when one reaches the top of a tree, it is easier to get a better view of the cricket match. Hence, option B is correct.
The preposition, 'on' is used to indicate the place or position of a noun relative to another noun. Eg: 'The apple is on the table'; 'People sat on the top of the tree'. It cannot indicate the direction of climbing. Hence, option A is incorrect.
The preposition, 'upon' is a combination of the words, 'up' and 'on'. 'Climbed upon something' means 'climbed up in order to be on something'. Given this meaning, when this preposition is used in the blank, it forms a meaningful sentence. Hence, option C is also correct.
The phrase, 'climb over something' means 'go up over something and then come down on the other side'. In the given sentence, the people climbed up the tree and did not come down on the other side. Hence, option D is not appropriate.